tvOS 11.3 was developed for the fourth and fifth generation of the Apple TV. The new version can be easily installed via the settings of the set-top box from now on rden.
Earlier tvOS 11.3 updates introduced support for AirPlay 2, so the Apple TV can be added to the Home app as part of a HomeKit setup, and multiple Apple TVs in separate rooms can play the same song, but this functionality has been added to tvOS 11.3 Beta 3 removed for unknown reasons.
Unfortunately, Apple did not re-install the AirPlay 2 features in tvOS 11.3. Some buyers of the HomePod are annoyed by the feature limitation that was actually promised and would have brought the smart speaker significantly more value. But postponed is certainly not lifted.
Other new features in tvOS 11.3 include improvements in auto-refresh rate on the fourth-generation Apple TV. In an earlier tvOS update already got the Apple TV 4K this feature. There is also an automatic mode switch for AirPlay video sessions.