Similar to the recent watchOS 2.2 Beta 2, the third build of watchOS 2.2 is a minor update featuring:
Performance improvements
Under-the-hood enhancements
Bug fixes
watchOS 2.2 Beta 2 is now available through Software Update from an iPhone running iOS 9.3 Beta, by going to the Apple Watch app > General > Software Update, or from the Watch Dev Center for registered devs.