You can find this and other legacy releases on the official Sourcetree Download Archives . Why You Need a Specific Version

If you are running , you may have noticed that the latest version of Sourcetree requires a newer operating system. However, you can still use this powerful Git client by downloading a compatible legacy version. The Best Version for macOS 10.13.6

Sourcetree 3.x was specifically optimized for High Sierra, including support for Swift 4.