Preview at WWDC likely in June, followed by September launch.
Apple Seeds Second OS X 10.11.3 El Capitan Beta to Developers and Public Beta Testers
The second OS X 10.11.3 beta, build 15D13b, can be downloaded through the Apple Developer Center or via the Software Update Mechanism in the Mac App Store for those who installed the first beta.
The first beta of OS X 10.11.3 did not include any noticeable outward-facing changes, suggesting the update will focus on security enhancements, performance improvements, and bug fixes to address issues that have been discovered since the release of OS X 10.11.2. Prior OS X El Capitan updates have also focused mainly on under-the-hood improvements.
We'll update this post with any changes that are discovered in the beta.