http://blogs.windows.com/windows_phone/b/windowsphone/archive/2013/10/14/announcing-our-third-windows-phone-8-update-plus-a-new-developer-preview-program.aspx
Support for bigger, higher-resolution screens
So the new update paves the way for future Windows Phone devices with 5- and 6-inch touch screens. The larger, 1080p HD displays on these devices will make Windows Phone even more personal—for example by sporting jumbo-sized Start screens with room for six Live Tiles across instead of four.
More powerful hardware
In addition to larger screens, Update 3 will also bring support for the Qualcomm Snapdragon 800 quad-core processor. The added horsepower that this chip delivers should make our already-fluid operating system perform even better.
Driving Mode
A new feature called Driving Mode helps you get from point A to point B with fewer distractions. Working with a connected Bluetooth device, Driving Mode is designed to limit notifications on the lock screen—including texts, calls, and quick status alerts—until you’re safely parked.
Other Features
- More useful ringtones: With Update 3, you can use custom ringtones for more things—including instant messages, emails, voicemails, and reminders. You can also assign custom ringtones to contacts for text messages, so you'll know who's texting you without even looking.
- No more twist and shout: Does your screen keep spinning when you’re trying to read emails in bed? Use the new rotation lock option to keep it fixed in place.
- Better storage management: New storage settings make it easier to free up space on your phone and manage temporary files. A new category view shows what's taking up space at a glance.
- Easily close apps: Now you can use the App switcher to quickly close apps when you're finished with them.
- Wi-Fi access out of the box: You can now connect to Wi-Fi during phone set up, so you can start conserving cellular data right out of the box.
- Better Bluetooth: The team made a bunch of improvements to improve connection quality for Bluetooth accessories.