Installing stock Android on Galaxy S3 fails. How to see why?
You can't install the stock ROM zip like as custom rom. Extract the stock rom zip and you will get the .tar PDA file to flash via ODIN. Download ODIN 1.85 and also install Samsung USB driver and then reboot your phone in download mode by pressing three combo key VOL Down+ Home+ Power button. Now connect it to pc while ODIN is opened on PC. Now your phone should show up on some port and usind PDA button and browse the firmware you extracted from the zip on pc and select it and flash. You will loss the root by doing this and also all data will be wipe out.