https://wccftech.com/apple-releases-third-public-beta-of-tvos-15-and-watchos-8/

Apple has released beta 3 of its upcoming tvOS 15 and watchOS 8 to the public. The new beta builds arrive two weeks after the initial beta was released. If you are enrolled in Apple's public beta testing program, you can install the latest builds on your compatible Apple TV models and Apple Watch models right now.

As mentioned earlier, you can install the latest public betas right now on your compatible devices, Make sure that the proper configuration profile is installed from Apple's beta testing website.