Registered and paid developers can get the new iOS 11.3 Beta 4 in Developer Download center or over-the-air as soon as the corresponding configuration profile has been installed from Developer Center.
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 for the iPhone X and a supplement to the Health App. Since the iOS 11.3 Beta 2, there are also functions for displaying the state of the act and the possibility of withdrawing any throttling of the system. 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
iBooks had lost the "i" from its name in the previous betas, but now it's back. Why Apple has moved here, remains unknown. iOS 11.3 will be released in the spring, according to Apple. Spring will start on March 20, so there's plenty of time for another beta.
Presumably there will soon be iOS 11.3 Beta 4 for public beta testers.