Unfortunately, it looks like the ATT version is the I747. It includes quad band gsm, 700/1700 lte, and 850/1900/2100 umts. No 1700, so you'd be stuck with edge.
The only sgs3 that comes with aws umts is the t999, which is the tmobile version (also mobilicity, wind, videotron in canada), but that version does not have the lte radio. Looks like the other sgs3's don't have aws umts at all. The Japan model (sc-06d) has band ix 1700 (incompatible with american 1700 aws), and 800mhz lte (probably not compatible with american 800).
Tis a real bummer.