A JTAG is a method used to fix software hard bricks. This is not something the average person can do because it requires expensive hardware and knowledge of microelectronics. These people specialize in doing so, but you will have to pay for it and send your device to them. However they have an extremely high success rate at reviving any phone brick not caused by hardware failure.
eh save the money the gs3 has 1 year factory warranty and they will fix bricked devices im an old customer that has sent two bricked devices each with warranty and they sent me a working device no question asked. just get ahold of the samsung technical support number and tell them u have a phone that wont turn on. they will try assisting u in trying to start it up recovery mode or download mode once they see it doesn't do anything they will send u a prepaid shipping label.