Can i put the sim card from my iPhone 4 into a Galaxy s3 for ATT?

I have an ATT iPhone 4 now. I want to buy a Galaxy s3 for ATT. Can use my sim for the new phone?

Technically you can (as in it will work just fine doing that), but AT&T might frown upon it. However I would say go ahead because you should be able to use your phone that you paid for, the way that YOU want. Hope this helps!