Designed for the fourth and fifth-generation Apple TV models, the tvOS 11.3 developer beta can be downloaded onto the Apple TV via a profile that's installed using Xcode and Apple Configurator.
The tvOS 11.3 update introduces support for AirPlay 2, allowing the Apple TV to be added to the Home app as part of a HomeKit setup.
With AirPlay 2, you can play a song on multiple Apple TVs in different rooms, and when AirPlay 2 officially launches, that same functionality will extend to other AirPlay 2 devices like HomePod and AirPlay 2 compatible speakers.
Other new features in tvOS 11.3 include enhancements to Match Content support, automatic frame rate switching on the fourth-generation Apple TV, and automatic mode switching for AirPlay video sessions.
Update: tvOS 11.3 beta 2 fixes a bug that caused the Play/Pause button on the Apple TV Siri Remote not to function properly.