Preview at WWDC likely in June, followed by September launch.
Apple Seeds Second iOS 9.3.3 Beta to Public Beta Testers
Beta testers who have signed up for Apple's beta testing program will receive the iOS 9.3.3 update over-the-air after installing the proper certificate on their iOS device.
Those who want to be a part of Apple's beta testing program can sign up to participate through the beta testing website, which gives users access to both iOS and OS X betas.
As another minor 9.x.x update, iOS 9.3.3 includes under-the-hood bug fixes and performance improvements to address issues that have been discovered since the release of iOS 9.3.2. No outward-facing changes or immediately apparently bug fixes have been discovered in the first two developer betas of iOS 9.3.3.