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.
Early tvOS 11.3 updates introduced support for AirPlay 2, allowing the Apple TV to be added to the Home app as part of a HomeKit setup and enabling multiple Apple TVs in separate rooms to play the same song, but that functionality was removed in tvOS 11.3 beta 3.
It is not yet clear if Apple plans to re-add AirPlay 2 features ahead of the release of tvOS 11.3, but it's a possibility because there are still several betas to go.
Other new features in tvOS 11.3 include enhancements to Match Content support, automatic frame rate switching on the fourth-generation Apple TV (a feature that was added to Apple TV 4K in a past tvOS update), and automatic mode switching for AirPlay video sessions.
Update: Apple has also seeded a fourth beta of tvOS 11.3 for public beta testers.