Ok so i have been researching and i'm soooo confused. I am on 4.3. I have read that i need to downgrade but doing so will hard brick my phone. I need to sim unlock it so i can use it on cincinnati bell. can somone please tell me how i can sim unlock my phone successfully? When i go into the hidden menu i have the option that says rat_free_support.
Unfortunately, nobody has yet posted a method to successfully free unlock the S3 running on 4.3. It's possible to downgrade your ROM only to 4.1.1 without downgrading the bootloader and retain full phone functionality. However, you would still not be able to free unlock as you need to be on complete stock 4.1.1, including the bootloader, to be able to free unlock. Until someone comes up with an exploit, your best options now would either be to request the unlock code from your carrier, if eligible, or purchase an unlock code online.
Downgrading 4.1.2 to 4.1.1 and unlocking is a breeze. You will find the Odin flashable file and downgrading instructions via the link below...