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.