Jailbreak your device using one of these tutorials.
Launch Cydia from your SpringBoard.
Press to choose Sections from the tabs at the bottom of the screen.
Select System from the list of Sections.
Choose iFile from the list of Packages.
Press the Install button at the top right of the screen. A free trial is available if you haven't already purchased this app.
Press the Confirm button to begin installation.
Press the large Return to Cydia button once installation has completed successfully.
Press the Home button to return to your SpringBoard then launch iFile
Navigate to /System/Library/CoreServices/SpringBoard.app/.
Press to select N90AP.plist from the list of files then choose Text Viewer from the popup menu.
Press the Edit button at the top left of the screen.
Add the following:
inside the capabilities dict, then press the Save button:
You can now restart your iOS device and and use FaceTime over 3G. (Don't forget to boot tethered if you need to).