UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, DC 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the

Securities Exchange Act of 1934

Date of report (Date of earliest event reported): February 7, 2020

Golub Capital BDC 3, Inc.

(Exact name of Registrant as Specified in Its Charter)

200 Park Avenue, 25th Floor, New York, NY 10106

(Address of Principal Executive Offices)          (Zip Code)

Registrant’s telephone number, including area code: (212) 750-6060

(Former Name or Former Address, if Changed Since Last Report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

Securities registered pursuant to Section 12(b) of the Act:

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 or Rule 12b- 2 of the Securities Exchange Act of 1934.

Emerging growth company þ

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. þ

On February 7, 2020, Golub Capital BDC 3, Inc. (the “Company”), together with GBDC 3 Funding II LLC, a direct, wholly owned subsidiary of the Company, as borrowers, entered into an amendment (the “SB Revolver Amendment”) to the documents governing the revolving credit facility dated as of February 4, 2019 by and among the Company and Signature Bank, as administrative agent and a lender (as amended, the “SB Revolver”). The SB Revolver Amendment was effective as of February 7, 2020.

The SB Revolver Amendment amended the SB Revolver to, among other things, increase the borrowing capacity under the SB Revolver to $275.0 million from $225.0 million and increase the borrowing base against which the Company may borrow through April 7, 2020, after which the borrowing base will revert to the terms applicable to the Company prior to the SB Revolver Amendment. The Company also paid a facility fee equal to approximately 0.2% of the increased commitment amount under the SB Revolver. The other material terms of the SB Revolver were unchanged.

The description above is only a summary of the material provisions of the SB Revolver Amendment and is qualified in its entirety by reference to a copy of the SB Revolver Amendment, which is filed as Exhibit 10.1 to this current report on Form 8-K.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, Golub Capital BDC 3, Inc. has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.