How to tell the difference between an S-3A and S-3B
By Scott Van Aken (ex-S-3 tech)

Basically there are three easy external clues, all visible from the left side. They are (from the front): 

an extra vent in front of the APU

square wing tip pod antennas

chaff/flare dispenser

And now you know. There is also a third chaff/flare dispenser that is located between the small wheel well doors.