Ahh.. if it's just 3-prong, then there's probably no neutral in the dryer outlet box after all -- so no 120V access there.

If it's working, leave it. Otherwise they might have to run new wiring to it for that fourth prong. Expensive.

EDIT: On the other hand, if/when the current dryer dies, the replacement will most likely demand a 4-prong outlet.. leaving you in the lurch until it gets rewired then.

Cheers