The macOS Sierra 10.12.3 update is available for download through the Apple Developer Center or through the software update mechanism in the Mac App Store.
According to Apple's release notes, the 10.12.3 update "improves the stability, compatibility, and security of your Mac." No specific changes, bug fixes, or feature additions were discovered in the first three betas, but we'll update this post should any new features be found in the fourth beta.
With bug fixes, it's often hard to determine exactly what's included in an update until Apple supplies detailed release notes with a release. We do, however, know that this update fixes a Safari bug that caused battery life problems during the MacBook Pro testing conducted by Consumer Reports.
Available since September, macOS Sierra is the latest Mac operating system. It includes Siri support, Apple Pay for the web, Universal Clipboard, Apple Watch auto unlocking, improved iCloud Drive integration, Picture-in-Picture multitasking, and dozens of smaller features that can be found in our macOS Sierra roundup.
Update: The fourth macOS Sierra 10.12.3 beta is also available for public beta testers.