google "Samsung Galaxy S3 toolkit 2.0" download it. Once downloaded flash custom recovery. once you've flashed custom recovery, download cmw superSU and put it on your sd card. Then boot into custom recovery and by holding volume up, home button and power button all at once. Then install zip from sd card and find, 'cmw supersu". when its finished reboot and you've rooted tmobile gs3 on jelly bean.