Help Can I flash the i747 (at&t) rom onto a t999 (tmobile)

Hi... new here and I know that what I want to know has probably been answered somewhere before, but what I read so far has all been conflicting. So I figured and hope to get answers to my questions, specifically.

So, here's the deal: I just recently broke my primary phone so I bought a t999 version of the gs3 to use with straight talk. I'd heard that tmobile technology was compatible with att's. From what I've read, the radio frequencies set to the rom of each version is different. I was thinking somehow flash att's radio frequencies to pick up on the internet. I'm stuck in 2g and that just wont fly. I'm afraid of hard bricking it, so I also thought that maybe flashing att's whole i747 rom onto it might be a safer route in attempting to pick up the radio signals for 4g. I've already talked to straight talk and the girl told me that my gs3 wasn't 4g CAPABLE. She mustve been high ha. Can i safely flash it? Is there a way to ever get this t999 to pick up at&t's radio? Step by step help would be appreciated, if any is available. Please and thanks.