The update is available via Software Update on your device or via the iOS dev center for iPhone, iPad and iPod Touch. It weighs in at 388MB for the iPhone 5s.
We've put together a lit of what's new in iOS 8 Beta 3 here.
There's many more changes in iOS 8. Here's a few of the changes we've posted on already: Continuity, Handoff, AirDrop, Instant Hotspot, Health, iCloud Drive, Family Sharing, Photos, Camera, Mail, Safari, Messages, QuickType, Third-Party Keyboards, Interactive Notifications, Spotlight, and Contact Shortcuts.
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