Here’s a list of Galaxy S3 stock firmware you can download. These are stock firmwares you can use ODIN to turn your Galaxy S3 completely back to factory. If you just need to return your phone back to stock ROM with root, you can also install Galaxy S3 stock ROMs instead.
U.S. Galaxy S3 stock firmwares
AT&T Galaxy S3 SGH-i747 Stock Firmware
*Note – For those of you on older bootloader, once you upgrade to 4.3 or 4.4.2, you cannot install AOSP custom ROMs!!!
T-Mobile Galaxy S3 SGH-T999 Stock Firmware
T-Mobile Galaxy S3 Stock Firmware NC2 [Android 4.3][Latest]