When did a Galaxy S3 become a Nexus phone??? We were discussing Nexus devices.I found rooting a Galaxy S3 to be a rather non-trivial process. There are a number of steps to follow. I managed to get through it all. It took me 15 minutes as I kept checking each step to make sure I'm not doing it wrong.
Most people use Odin for Samsung devices, which is slightly more complex than using fastboot commands to unlock/root/rom.