Apple has released the third beta version of iOS 9 to users registered in the Apple Developer program, arriving as build 12A4293f. Separately, WatchOS 2 beta 3 is also available to developers.
The 3rd developer beta version supports all of the same iOS 9 compatible iPhone, iPad, and iPod touch models. The release can be downloaded now from the Over the Air software mechanism on a device running a prior beta release, or the firmware can be downloaded through the Apple Developer center website as IPSW files to be installed manually from iTunes.
Do note this version is limited to registered Apple developers, it is not yet part of the iOS Public Beta program, which is scheduled to open in the coming weeks for iOS 9 betas.
Additionally, WatchOS 2 beta 3 is available to users participating in the developer beta program for Apple Watch. The WatchOS release also requires an iPhone running iOS 9 beta 3, but because the Watch does not have a user serviceable USB port, there is no way for users or developers to restore an Apple Watch running a beta version of WatchOS back to a stable version without giving the device back to Apple, WatchOS betas are truly only recommended for those who are building apps for the Watch platform. This contrasts dramatically to iOS 9 betas, which can easily be downgraded back to a stable iOS 8 release by connecting the beta running iPhone or iPad to iTunes via USB and using the restore mechanism.
Aside from the mobile world, Mac users will find OS X El Capitan developer beta 3 is also available.
iOS 9 and WatchOS 2 are expected to debut in the fall with a variety of new features and performance improvements, quite likely alongside a new iPhone model.