Apple today released the third developer betas of iOS 11.3, macOS 10.13.4 High Sierra, and tvOS 11.3, exactly two weeks after the second build was offered. These are mid-stream feature updates, so they're not quite the minor updates that the version numbers indicate.
As usual, iOS 11.3 is going to be the most significant update. iPhone X users will get new Animojis, but users will also be able to find out if their device is being throttled due to an old battery. There's also Business Chat in Messages, giving people the ability to connect directly with a representative from a business. Messages in iCloud is a feature that was promised when iOS 11 was first announced, but it's only being delivered now.
This particular build, 15E5189f, has some known issues, such as the new Health Records feature not syncing to iCloud. Apple has fixed the issue causing Maps to crash though, and managed books now work in iBooks.
As for macOS 10.13.4 High Sierra, users will begin receiving notifications that 32-bit apps will no longer be supported in a future update, which is likely macOS 10.14. A new issue in build 17E160e is that recovery over Wi-Fi won't work on iMac Pros.
As for tvOS 11.3, new features include enhancements to Match Content, automatic frame rate switching for the fourth-generation Apple TV, and automatic mode switching for AirPlay. All of the known issues in build 15L5186e have to do with mode switching.
All of these updates, including watchOS 4.3, will be available this spring.