Do they iphone 4 and Samsung Galaxy S3 have the same data plans for AT&T?

Would I just be able to buy a Galaxy S3 and put my sim card into it and everything will be okay?
Or do they have two different data plans?

Yes, just everytime you log on to your account will say you have a iPhone until you change it

If it's an AT&T Galaxy S3, and the SIM card is the same size, then yes, you can just plug it in and it should be fine.