Apple seeded macOS High Sierra developer beta 3 on Monday, dubbed macOS High Sierra 10.13 beta 3 (17A306f). In this early phase in the developer beta cycle, Apple is using each release to squash bugs, tweak the interface, and try different approaches in new features.
Registered developers can find the update at the Apple Developer Connection. Devices that have already been provisioned for the developer beta cycle can find the update in Software Update in the Mac App Store.
This release does not affect those who are running the public beta of macOS High Sierra, which is still on its first release.