Q: Can I use iphone 3g in iphone 4 or 5

The iPhone 3G uses a full-sized SIM card.

The iPhone 4 and and 4S uses a MicroSim card.

The iPhone 5 and greater uses a nanoSim card.

http://support.apple.com/kb/HT5554

Go to an AT&T retail store and request the appropriate size SIM for the device you desire to use.  This does not affect your contract and they generally will give customers a new SIM at no charge.

Actually, the 3G, 3GS, and original iPhone, along with most other GSM phones from that era, use a Mini SIM.

A "full sized SIM" card is (was actually, since nothing uses them any more) the size of a credit card! Gaak!

If your phone doesn't look like this, you probably don't have a full sized SIM:

http://en.wikipedia.org/wiki/Subscriber_identity_module#Full-size_SIM

(just an interesting piece of trivia thrown out there for the sake of picking nits )

Remember those. They looked like portable CIA encryption kits....