Preview at WWDC likely in June, followed by September launch.
Apple Seeds Third Beta of iOS 9.3.3 to Developers and Public Beta Testers
The third iOS 9.3.3 beta can be downloaded from the Apple Developer Center or over-the-air with the proper configuration profile installed.
As a minor 9.x.x update, iOS 9.3.3 features under-the-hood bug fixes and performance improvements to address issues discovered since the release of iOS 9.3.2. No outward-facing changes or obvious bug fixes were discovered in the first two betas of iOS 9.3.3.
iOS 9.3.3 beta three follows the developer launch of iOS 10, a new version of iOS that will be released to the public this fall. We'll update this post with any changes that are found in the third iOS 9.3.3 beta.