So I haven't been on a S3 for a little while but I do remember. In fact if you want to downgrade the AT&T S3 (SGH-I717) from 4.3 to lower firmwares such as 4.1.1 for free unlocking download this:
http://terafile.co/407413755110/I747UCDLK3_I747ATTDLK3_I747UCDLK3_HOME.zip which will be the stock 4.1.1 ROM. I REPEAT DO NOT FLASH ANY BOOTLOADERS AS IT WILL RESULT IN A HARDBRICK!!! THE ZIP ONLY CONTAINS THE OS SO YOU'RE GOOD. You'll need Odin, I prefer 3.07 because the new 3.09 changes the button names. You can download Odin from here:
http://xdaforums.com/attachment.php?attachmentid=1168421&d=1341114315
Alright so on to the downgrading. (this will wipe everything so back up before hand)
"I'm not held responsible for any damage to your device. You have been WARNED"
1. Turn off the device and go into download mode by holding the power button and volume up, you should be greeted with some description press volume up to continue
2. Plug your device into your computer and open Odin.
3. Click PDA and search for the ROM you dowloaded. (Make sure you extract the ROM out of its zip form to get the .tar)
4. One selected in PDA press start and sit back and enjoy your free unlock
this can be used for any stock OS just remember to not flash any bootloaders
Side Note: It takes a while to flash and also reboot. If the phone doesn't reboot do a factory reset through the recovery and it should boot. I believe this will also work for other variants of the S3 as long as you have the right stock image
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