Updated 04/17/18: Apple has released iOS 11.4 beta 2 to the public. It comes two weeks after beta 1, and focuses on fixing bugs, with no major changes to features discovered yet.
iOS 11.3 was released on March 29 after more than two months of beta testing. It added several new features, including battery health and performance settings, four new Animojis, Business Chat and Health Records betas, and a bit upgrade to ARKit.
Two big features we've been waiting on since the release of iOS 11 last year were notably absent: AirPlay 2 and Messages in iCloud. Both features were included in beta, but were cut from the final release. Those features are unlikely to get pushed back to iOS 12 (due for release this fall), which means iOS 11 will have yet another point release.