The tvOS 11.2.5 public beta can be obtained by going to the Settings app on the Apple TV and navigating to the Software Updates section under "System." "Get Public Beta Updates" will need to be toggled on, and once it is, the Apple TV will download the beta software.
tvOS 11.3 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 music 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.