Yesterday, Apple released new Developer Beta builds of iOS 10.2.1 and macOS 10.12.3 Sierra, but watchOS has notably been missing from this round of betas, until today. The company has seeded watchOS 3.1.3 Beta 1 to developers.
As the version number indicates, there are no new front-facing features in the build, which has the number 14S958. In fact, watchOS hasn't seen any new features since version 3.1 was released in October.
Naturally, you might be wondering what happened to watchOS 3.1.2. It would seem that the company has skipped it entirely, as the last update was 3.1.1. The company ended up having to pull that update though, as it was breaking some Apple Watch Series 2 devices, so it's possible that watchOS 3.1.2 might still roll out as a replacement update while 3.1.3 is still in beta.
Like tvOS, there is no Public Beta for watchOS. If you want access to today's build, you'll need to be a registered developer. Once you are, you can download the configuration profile from Apple Developer to your Watch, and then update through the Watch app on your iPhone.