iOS 11.3 is a major update that includes many new features, such as: Messages in the iCloud to synchronize messages across devices, and ARKit 1.5, a new version of ARKit that is able to more accurately map irregularly shaped surfaces, recognize images, and place objects on vertical surfaces such as walls.
In addition, there are four new Animoji and a supplement to the health app for the iPhone X. Since the release of the iOS 11.3 Beta 2, there are also battery-health indicators and the ability to take back system throttling. This only applies to iPhones - no throttling has been done on iPads.
The betas of iOS 11.3 and tvOS 11.3 included support for AirPlay 2 until recently, but this new feature was released in iOS 11.3 and tvOS 11.3 Beta 3 removed again. iBooks had lost the "i" from its name in the previous betas, but since beta 4 it's back. Why Apple changed the course here has not been communicated so far. iOS 11.3 will be released in the spring of 2018, according to Apple. Spring is starting on March 20, so that's definitely time for another beta, if you take it very seriously
Surely, iOS 11.3 beta 6 will soon be available for public beta testers, possibly in the course of time of the evening.