Apple has not revealed any changes in this release (build 9B5141a) other than that is fixes bugs and improves on the security. The Beta 3 follows the Beta 2 release on December 12.
The update which is currently not available on the developer portal can only be obtained OTA and is 76.3MB in size. So if you are currently running iOS 5.1 Beta 2, head over to Settings > General > Software Update and see if the update is available. Make sure you are connected to Wi-Fi to avoid cellular data charges.
[UPDATE] It has been reported that the new build restores the 'Enable 3G' toggle switch. The release notes and the beta 3 have still not been made available on the Apple Developer Site.
[UPDATE] iOS 5.1 Beta 3 is finally available on the iOS Developer site along with the changelog in the update. XCode 4.3 Developer Preview 3 and Apple TV Software Beta 2 have also been released today
You will have to sign up for the Apple Developer Program to get access to this and many other beta products which are released to developers only. Enrolling for this program will cost you $99 a year. You are advised to avoid downloading these files from unofficial sources like torrent or file sharing sites. Downlaoding and installing unofficial files may compromise the functionality of your device.
source - MacRumors