iOS 9 Beta 3 features features bug fixes, under-the-hood improvements, and an updated Music app, which will allowing iDevices running iOS 9 to access the new Apple Music streaming service as well as the Beats 1 radio station.
You can get iOS 9 Beta 3 through Software Update or via the iOS Dev Center for registered devs, while registered public beta testers are scheduled to begin receiving iOS 9 Betas starting this month.