The new tvOS 11.3 public beta can be downloaded 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.
Earlier versions of tvOS 11.3 introduced support for AirPlay 2, allowing the Apple TV to be added to the Home app as part of a HomeKit setup and enabling multi-room audio across multiple Apple TVs.
In tvOS and iOS 11.3 beta 3, all AirPlay 2 functionality has been removed. It is not yet clear if Apple plans to re-add these features to the beta, so it remains to be seen if AirPlay 2 will be coming in iOS 11.3.
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 update), and automatic mode switching for AirPlay video sessions.