I don't know for sure, because I haven't tried it, but it may depend on how you do the USB tethering. ClockworkMod Tether claims to be undetectable and I haven't heard of anybody getting caught while using it. On the other hand, lots of people have gotten caught using wireless tethering.
What app were you using when you got caught?
This didn't happen on the GS3 but while I was using my previous phone, the HTC Incredible S. I was using the stock Android tethering feature via the stock unbranded WWE rom. I did this last year some time after AT&T announced that they were going to "crack down" on people who tether without the proper data plan.
Thanks for the info about the ClockworkMod Tether app. I'll have to take a look at it to see if it's something I could use and avoid detection.