Preview at WWDC likely in June, followed by September launch.
Apple Seeds Third Beta of macOS Sierra 10.12.3 to Developers
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 two betas, but we'll update this post should any new features be found in the third beta.
Oftentimes, with bug fixes, it's hard to tell what's included until Apple supplies detailed release notes with a release. The previous update, macOS 10.12.2, introduced several important bug fixes to resolve battery life issues, fix graphics problems, and more.
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 third public beta is also available.