| Cover | Feb. 02, 2024 |
| Document Information [Line Items] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001715268 |
| Document Type | 8-K |
| Document Period End Date | Feb. 02, 2024 |
| Entity Registrant Name | GOLUB CAPITAL BDC 3, INC. |
| Entity Incorporation, State or Country Code | MD |
| Securities Act File Number | 814-01244 |
| Entity Tax Identification Number | 82-2375481 |
| Entity Address, Address Line One | 200 Park Avenue |
| Entity Address, Address Line Two | 25th Floor |
| Entity Address, City or Town | New York |
| Entity Address, State or Province | NY |
| Entity Address, Postal Zip Code | 10166 |
| City Area Code | 212 |
| Local Phone Number | 750-6060 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | true |
