Exhibit 99.3

UNAUDITED PRO FORMA CONDENSED COMBINED FINANCIAL INFORMATION

The accompanying Unaudited Pro Forma Condensed Combined Statement of Operations for the year ended December 31, 2014 and for the three months ended March 31, 2015 and the Unaudited Pro Forma Condensed Combined Balance Sheet as of March 31, 2015 and the notes thereto of Builders FirstSource, Inc. (the “Company”) (“Unaudited Pro Forma Condensed Combined Financial Information”) have been derived by the application of pro forma adjustments based upon the historical financial statements of the Company and ProBuild Holdings, Inc. (“PBHI”), after giving effect to the ProBuild Acquisition, ABL Facility, First Lien Term Loan Facility, the Senior Unsecured Notes (and the Senior Unsecured Bridge Facility if drawn in lieu of the issuance of Unsecured Notes) (each as defined below), and issuance of the Company’s common stock (collectively, the “Transactions”) and related adjustments described in the following notes, and are intended to reflect the impact of the Transactions on the Company on a pro forma basis.

The final terms of the Transactions will be subject to market conditions and may change materially from the assumptions described in the following Unaudited Pro Forma Condensed Combined Financial Information. Changes in assumptions described below with respect to the ABL Facility, First Lien Term Loan Facility, the Senior Unsecured Notes, and the issuance of common stock in connection with the ProBuild Acquisition financing would result in changes to various components of the Unaudited Pro Forma Condensed Combined Balance Sheet, including long-term debt and stockholders’ equity, and various components of the Unaudited Pro Forma Condensed Combined Statement of Operations, including interest expense and earnings per share. Depending upon the nature of the changes, the impact on the Unaudited Pro Forma Condensed Combined Financial Information could be material.

The final purchase price allocation for the Transactions will be performed after the closing of the ProBuild Acquisition and will depend on final asset and liability valuations, which may depend in part on prevailing market rates and conditions. These final valuations will be based on the actual net tangible and intangible assets that exist as of the closing of the ProBuild Acquisition. Any final adjustments may change the allocations of purchase price, which could affect the fair value assigned to the assets acquired and liabilities assumed, and could result in a change to the Unaudited Pro Forma Condensed Combined Financial Information, including goodwill. The result of the final purchase price allocation could be materially different from the preliminary allocation set forth herein.

The Unaudited Pro Forma Condensed Combined Financial Information has been prepared to reflect adjustments to the Company’s historical consolidated financial information that are (i) directly attributable to the acquisition, (ii) factually supportable and (iii) with respect to the Unaudited Pro Forma Combined Statement of Operations, expected to have a continuing impact on the combined results. The differences between the actual valuations and the current estimated valuations used in preparing the Unaudited Pro Forma Condensed Combined Financial Information may be material and will be reflected in the Company’s future balance sheets and may affect amounts, including depreciation and amortization expense, which the Company will recognize in its statement of operations following the acquisition. The Unaudited Pro Forma Condensed Combined Financial Information is not intended to represent or be indicative of the consolidated results of operations or financial position of the Company that would have been reported had the Transactions been completed as of the dates presented, and should not be taken as representative of the future consolidated results of operations or financial position of the Company. The Unaudited Pro Forma Condensed Combined Financial Information does not reflect any operating efficiencies and cost savings that the Company may achieve with respect to combining the companies or costs to integrate the business. Synergies and integration costs have been excluded from consideration because they do not meet the criteria for unaudited pro forma adjustments.

The Unaudited Pro Forma Condensed Combined Financial Information is provided for informational and illustrative purposes only and should be read in conjunction with the Company’s historical financial statements and related notes thereto, included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2014 and the Company’s Quarterly Report on Form 10-Q for the three months ended March 31, 2015, and PBHI’s audited historical combined financial statements and notes and PBHI’s quarterly historical combined financial statements and notes thereto (which are available as Exhibits 99.1 and 99.2 to the Current Report on Form 8-K of which this financial information forms an exhibit, respectively). The audited combined financial statements and unaudited interim condensed combined financial statements reflect all of the operations of ProBuild Holdings LLC, the business to be acquired. Net liabilities, included in the combined financial statements of PBHI, not assumed in the ProBuild Acquisition are approximately $644.4 million as described in note 6(g).

The following Unaudited Pro Forma Condensed Combined Financial Information was prepared using the acquisition method of accounting for business combinations under the guidance in Accounting Standards Codification (“ASC”) Topic 805, Business Combinations. The Unaudited Pro Forma Condensed Combined Balance Sheet reflects the Transactions as if they had been consummated on March 31, 2015 and includes pro forma adjustments for preliminary valuations of certain tangible and intangible assets by management in accordance with the acquisition agreement dated April 13, 2015. The Unaudited Pro Forma Condensed Combined Statement of Operations for the year ended December 31, 2014 combines the Company’s historical results for the year ended December 31, 2014 with PBHI’s historical results for the year ended December 31, 2014 and the Unaudited Pro Forma Condensed Combined Statement of Operations for the three months ended March 31, 2015 combines the Company’s historical results for the three months ended March 31, 2015 with PBHI’s historical results for the three months ended March 31, 2015. The Unaudited Pro Forma Condensed Combined Statement of Operations gives effect to the Transactions as if they had been consummated on January 1, 2014.

The Unaudited Pro Forma Condensed Combined Financial Information has been compiled in a manner consistent with the accounting policies adopted by the Company. These accounting policies are similar in most material respects to those of PBHI. Following the ProBuild Acquisition, the Company will perform a more detailed review of PBHI’s accounting policies. As a result of that review, differences may be identified between the accounting policies of the two companies that, when conformed, could have a material impact on the combined financial statements.

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Builders FirstSource, Inc.

Unaudited Pro Forma Condensed Combined Statement of Operations

For the Three Months Ended March 31, 2015

3

Builders FirstSource, Inc.

Unaudited Pro Forma Condensed Combined Statement of Operations

For the Year Ended December 31, 2014

4

Builders FirstSource, Inc.

Unaudited Pro Forma Condensed Combined Balance Sheet

As of March 31, 2015

5

On April 13, 2015, the Company entered into a Securities Purchase Agreement (the “Agreement”) with ProBuild Holdings LLC, and the holders of securities of ProBuild named as parties thereto (collectively, the “Sellers”). Pursuant to the Agreement, the Company will acquire all of the operating affiliates of ProBuild through the purchase of all of the issued and outstanding equity interests of ProBuild for approximately $1.63 billion, subject to working capital adjustments and other adjustments (the “ProBuild Acquisition”).

The Company has received a debt commitment letter from the parties thereto and certain of their respective affiliates pursuant to which they have committed to provide the Company with an $800 million senior secured asset-backed loan facility (the “ABL Facility”), a $550 million senior secured first lien term loan facility (the “First Lien Term Loan Facility”), and a $750 million senior unsecured bridge facility (the “Senior Unsecured Bridge Facility”) (with respect to which the Company may issue up to $750 million in senior unsecured notes in lieu of all or a portion thereof (the “Senior Unsecured Notes”)). Additionally, the amount of the First Lien Term Loan Facility may be increased to repay, redeem, or defease the Company’s existing senior secured notes in an amount equal to $350 million plus any applicable make-whole payments required in connection with any such repayment, redemption, or defeasance. Under certain circumstances, the increase may take the form of senior secured notes or senior unsecured notes instead of additional first lien term loans. The proceeds of the borrowings will be used on the closing date of the ProBuild Acquisition to pay a portion of the aggregate acquisition consideration, to refinance certain of the Company’s existing debt and to pay related fees and expenses.

The Unaudited Pro Forma Condensed Combined Financial Information included herein has been prepared pursuant to the rules and regulations of the Securities and Exchange Commission. Certain information and certain footnote disclosures normally included in financial statements prepared in accordance with U.S. generally accepted accounting principles have been condensed or omitted pursuant to such rules and regulations; however, management believes that the disclosures are adequate to make the information presented not misleading.

Following the ProBuild Acquisition, the Company will conduct a review of accounting policies of ProBuild in an effort to determine if differences in accounting policies require reclassification of results of operations or reclassification of assets or liabilities to conform to the Company’s accounting policies and classifications. As a result of that review, the Company may identify differences among the accounting policies of the companies that, when conformed, could have a material impact on the Unaudited Pro Forma Condensed Combined Financial Information. The Company has completed a preliminary review of accounting policies for purposes of the Unaudited Pro Forma Condensed Combined Financial Information. During the preparation of the Unaudited Pro Forma Condensed Combined Financial Information, the Company identified a significant difference in accounting policy in regards to accounting for inventory. The Company accounts for the cost of inventory using the weighted average method, the use of which approximates the first-in, first-out method. PBHI accounts for the cost of inventory using last-in, first-out method. An adjustment has been made in the Unaudited Pro Forma Condensed Combined Financial Information to conform PBHI’s inventory cost policy to the Company’s.

The audited combined financial statements of ProBuild Holdings, Inc. reflect all of the operations of the business to be acquired by the Company. Net liabilities, included in the combined financial statements of PBHI, not assumed in the acquisition were approximately $644.4 million as described in footnote 6(g). Financial information presented in the “Historical ProBuild” column in the Unaudited Pro Forma Condensed Combined Statement of Operations represents the historical combined statement of operations of PBHI for the year ended December 31, 2014 and for the three months ended March 31, 2015, respectively. Such financial information has been reclassified or classified to conform to the historical presentation in the Company’s consolidated financial statements as set forth below. Unless otherwise indicated, defined line items included in the footnotes have the meanings given to them in the historical financial statements of PBHI.

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Reclassifications incorporated in the Unaudited Pro Forma Condensed Combined Statement of Operations for the year ended December 31, 2014:

Reference:

Reclassifications incorporated in the Unaudited Pro Forma Condensed Combined Statement of Operations for the three months ended March 31, 2015:

Reference:

Adjustment also represents incremental depreciation of production-related assets of $2.0 million and $0.4 million, based on the fair value of property, plant and equipment, for the year ended December 31, 2014 and for the three months ended March 31, 2015, respectively.

7

Depreciable property and equipment is expected to be amortized on a straight-line basis over an estimated average useful life of 7 years. Intangible assets are expected to be amortized on a straight-line basis over estimated useful lives of 15 – 20 years.

8

The Company estimates the weighted average interest rate on the new debt incurrence to be approximately 6.3%. A hypothetical 1/8% increase or decrease in the expected weighted average interest rate on financing incurred in connection with the Transactions, including from an increase in LIBOR (excluding the impact of the LIBOR floor), would increase or decrease interest expense on the Company’s financing by approximately $2.6 million annually.

9

Under the acquisition method of accounting, the total estimated purchase price is allocated to ProBuild’s net tangible and intangible assets based on their estimated fair values at the date of the completion of the acquisition.

Below is a preliminary estimate of the purchase consideration for ProBuild, adjustment to the historical book value for net assets not acquired and the allocation of the purchase price to acquired identifiable assets and assumed liabilities.

The purchase price allocation for the ProBuild Acquisition includes values assigned to certain specific identifiable intangible assets aggregating approximately $523.7 million. The trade name related intangible assets are valued at $254.7 million, which was determined by estimating the present value of future royalty costs that will be avoided due to the Company’s ownership of the trade names acquired. The customer-related intangible is valued at $269.0 million, which was determined by estimating the present value of expected future net cash flows derived from such customers.

Inventories are increased to their estimated fair value, which represents an amount equivalent to estimated selling prices less distribution related costs and a normative selling profit. This inventory adjustment is expected to be fully recognized in cost of sales in the first three months after the transaction. We expect this step-up in basis and amortization of this amount to have a negative effect on gross margins until fully recognized.

Upon closing, the purchase consideration will be adjusted for working capital levels and other adjustments as stipulated in the purchase agreement.

Upon completion of the fair value assessment, the Company anticipates that the final purchase price allocation will differ from the preliminary assessment provided above. Any changes to the initial estimates of the fair value of the assets and liabilities will be recorded as adjustments to those assets and liabilities and the residual amounts will be allocated as an increase or decrease to goodwill.

10

Deferred financing fees of $52.2 million have been recorded, with $7.5 million classified as other current assets and $44.7 million classified as non-current assets. The proceeds from the First Lien Term Loan Facility have been reduced by a $5.5 million original issue discount, which is presented as a reduction of the outstanding balance on the First Lien Term Loan on the balance sheet.

Deferred financing fees incurred in relation to the First Lien Term Loan Facility, ABL Facility and Senior Unsecured Notes will be amortized over the contractual term of such respective facilities. Amounts related to the original issue discount will be amortized over the contractual term of the First Lien Term Loan Facility.

Deferred financing fees of $3.4 million relating to ProBuild’s previous credit facility have been eliminated from other current assets ($2.2 million) and other assets, net ($1.2 million) with a corresponding decrease to retained earnings. No adjustment has been made to the Unaudited Pro Forma Condensed Combined Statements of Operations for these costs as they are non-recurring.

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