Will a Iphone 3g unlocked still work on AT&T?

Ok i got a iphone 3g that jailbroken and can run on T-Mobile.
I want to know if i can still use a AT&T Sim card in it and it will work for AT&T or do i have to deactivate the thing that is making it unlocked?

Of course. Unlocked means it's unlocked. It's not locked to any carrier. It can run any carrier that has sin cards