So I'm a retard and I installed the MK3 boot loader on my AT&T GS3 and accidentally deleted the OS. I can still boot into the recovery and download mode. I can't flash anything through Odin since the boot loader doesn't let me. Is there a way to fix this or is my phone completely bricked? If there is a way to fix this, please tell me how. Thanks.
It's not a complete brick if you are able to boot into download and recovery modes. Your best option would be to attempt to restore your OS to as near stock as possible. I'm not sure which one you mean by the MK3 boot loader? There is no MK3 for any the GS3 variants or did you mean MK5 which is for the Canadian variant? You may try the method posted in the thread below to attempt a restore to stock 4.3 (I'm presuming you were on 4.3 prior to the firmware failure) ...
If you don't have custom recovery installed, you would need to flash this via Odin. You may not be able to install ROMs via Odin but you should be able to flash custom recovery if you can get to download mode.