Preview at WWDC likely in June, followed by September launch.
Apple Seeds Fourth Beta of iOS 9.3.3 to Developers and Public Beta Testers
Today's iOS 9.3.3 beta update can be downloaded from the Apple Developer Center (developer only) or over-the-air with the proper configuration profile installed.
As another minor 9.x.x update, iOS 9.3.3 features under-the-hood bug fixes and performance improvements to address issues unearthed since the release of iOS 9.3.2. No outward-facing changes or obvious bug fixes were discovered in the first three betas of iOS 9.3.3.
iOS 9.3.3 beta 4 follows the developer launch of iOS 10, a new version of iOS that will be released to the public this fall. iOS 10 includes features like a redesigned Lock screen experience, a revamped Messages app with its own App Store, a Siri SDK for developers, a new Home app for controlling HomeKit devices, a redesigned Apple Music app, and more.