Earlier this week, Apple released the third developer betas for iOS 11, macOS 10.13 High Sierra, and tvOS 11, and then a couple of days later, the company seeded the public betas. Throughout all of this, watchOS 4 beta 3 was notably missing, until today, as build 15R5321h is now listed in Apple Developer.
Unfortunately, that's all we know. We're not actually seeing the update on our devices, despite Apple having announced it, and clicking on the Release Notes link in Developer gives us a message that says "403 Forbidden". Naturally, we can assume that the build will include a list of resolved and known issues, and it will perform a bit better than beta 2.
watchOS 4 will contain a number of new features for the OS, including three new watch faces, all of which you can see in the image above. The Siri watch face is meant to be more proactive about what you might want to see when you lift your watch. You can also now opt to get rid of the cluster of apps in favor of a list view.
Presumably, the build will be live shortly, as will the release notes. We'll keep you updated if there's anything worth noting. Remember, if you install a watchOS beta, there's no easy way to roll back to watchOS 3.