https://wccftech.com/download-tvos-15-and-watchos-8-beta-3-has-been-seeded-to-developers/

You can now go ahead and download tvOS 15 beta 3 and watchOS 8 beta 3 onto your Apple TV and Apple Watch, if you are a registered developer.

Apple has released tvOS 15 and watchOS 8 beta 3 to registered developers. If you have an Apple TV or Apple Watch running the recent beta software, then you can upgrade to the third pre-release iteration of it in a few clicks.