When you first install iOS 8 beta 3, you'll go through a setup that gives you the option to upgrade to iCloud drive or to keep using iCloud Documents and Data. You can choose to upgrade later to iCloud drive in Settings. There is also a new option to send app analytics to developers to help improve App Store apps.
In settings, you can now fine a toggle to enable or disable handoff. Handoff lets you seamlessly start tasks on one device and continue them on another device.
iOS 8 also brought some enhancements to Messages, with the ability to quickly send voice messages and the option to automatically delete/keep attachments. Beta 3 gives you the option to keep all attachments automatically.
There have also been some minor tweaks to the Weather app's UI. You can see the previous UI on the left, and the new UI on the right. iOS 8 now uses data from Weather Channel instead of Yahoo to provide more forecast information.
Apple brought third-party keyboards and a new QuickType predictive text feature to iOS 8. However if you do not wish to use QuickType, you'll be happy to know that there is now a toggle to disable the feature entirely.
T-Mobile's Wi-FI calling feature has also been enabled on iOS 8 Beta 3 (via 9to5mac).
Shared Photo Streams have also been renamed to Shared Album Events. Other minor changes include a tweaked 'clear' button in the notification center, a new Wallpaper, enhanced iCloud Account Settings and a way to dismiss notifications on the lock screen. If you spot anything else, please let us know!