tvOS 11.3 can be downloaded over the air through the Settings app on the Apple TV by going to System --> Software Update. Apple TV owners who have automatic software updates turned on will be upgraded to tvOS 11.3 automatically.
tvOS 11.3 introduces performance improvements and bug fixes for issues discovered following the release of tvOS 11.2.6.
The update introduces 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.
The TV app, designed to let users find and watch television shows and movies aggregated from entertainment apps, is expanding to Brazil and Mexico. Brazil is also gaining Siri support on Apple TV 4K and the fourth-generation Apple TV.
Early beta versions of tvOS 11.3 included 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.
AirPlay 2 functionality was, however, removed in the third beta of tvOS 11.3, eliminating these features. AirPlay 2 was not re-added during the beta testing period, so it's likely Apple is saving the feature for a future update.