This happens to me too (new unlocked S3)
I had also given up but then I figured out how to get past it by accident:
When it is stuck, just unplug the USB cable, then plug it back in, and ODIN happily kicks in and does the flash with a successful PASS result.
Done it quite a few times now.
Jsut a weird quirk of the S3 it seems (my old Note was fine).
BTW I tried different USB ports, nonhubs, cables, ODIN183, 187, 304 etc. All did same, until I found this trick, and BTW I am actually flashing via a hub now and it works fine with this trick.
Hope this helps.