https://wccftech.com/download-tvos-12-beta-3-now-available-for-apple-tv-developers/

Apple TV developers can now download tvOS 12 Beta 3 for their set-top box right away. The update is available over the air.

If you are an Apple TV 4K user, then the tvOS 12 update means a lot to you as it brings in the much-awaited Dolby Atmos support to the fold. This means that you will get an even more immersive audio experience provided that you have the right equipment at home to make it all happen. But right now, all that goodness is in beta and tvOS 12 Beta 3 has currently been seeded to all developers out there and you can download it over the air if you have a previous beta installed.