iPhone iOS 6 Beta 3 Download has been released, and you can Download iOS 6 Beta 3 from below, which is official link. Apple recently released the iOS 6 Beta 3, though it was released for developers point of view. So, its better you avoid updating your iPhone to prevent security and other issues.
Download iOS 6 Beta 3
You can download ios 6 beta 3 release from either the below download link or you can directly update your iPhone right from System Updates. This update will remain open till September, so after that you cannot update to iOS6 Beta 3 release. You can update when the stable release gets unveiled by apple.