There is no official Android Lollipop update for Galaxy S3, but you can update your AT&T Samsung Galaxy S3 SGH-1747 to Android Lollipop 5.1.1. You need to flash customer ROM provided by PACMAN. If you are not using the specified model, please don’t try. You may end up corrupting the software.
This process required a rooted device, and make sure the battery is charged enough (at least 60-70 percent). Backup all the data. You will have to wipe everything from the device. Now enable USB debugging from Settings —> Development Options —> USB debugging.
These are the new features that this update will bring to your device
Update Samsung Galaxy S3 SGH-1747 to Android Lollipop 5.1.1 PACMAN
Download the PACMAN Android Lollipop 5.1.1 custom ROM as well as GApps package. Save it on your computer, but don’t extract them.
Now transfer the ZIP file to the Phone’s SD Card using USB cable and then disconnect it. Switch off your device. Turn it on in recovery mode (using volume up + Home + Power button).
Take a Nandroid backup of the current system OS (in case you face any trouble) : Backup —> Backup and Restore on ClockworkMod recovery
Now wide data/factory reset, and then Wipe cache partition and dalvik cache from CWM recovery
Now Select install zip file from SD Card, and locate the Customer Android Lollipop ROM. Allow falshing process to complete
Go back to the recovery menu and reboot the device
It would take longer than usual to boot the device first time
This is how you update Samsung Galaxy S3 SGH-1747 to Android Lollipop PACMAN.
If you want to go back to the same OS, simple boot the device into recovery mode. Select the Backup and restore option. Restore the device, and you are done.