The 3.5L is a Single overhead cam engine. There is only 1 cam over each cylinder head. But being a V6, it does have two cams total. If it was dual overhead cam there would be a total of 4 cams.
Heres a cross section of the 3.5L, its not much help, but fun to look at:
Being a 24 Valve v6 means it has 4 valves per cylinder, 2 intake, 2 exhaust. (Interestingly enough, the 3.5L heads are very similar to the new and old hemi engines)
Like i said, technically the 3.5L is classified as a interference engine, but the chances of any damage from a failed timing belt is highly unlikely due to the design of the engine.
I had the tensioner self-destruct in my concorde with the 3.5L, heres what i saw under the timing cover:
and the new tensioner next to the old one:
anyways, back on track to your problem. It is possible there is damage to a cam gear? Yes i suppose it's possible, but i can't imagine how... It sounds as if the mechanic is trying to scam you. Do some more research on the subject, check out
www.allpar.com, lots of good info on the engine there.
btw, I replaced the timing belt in my car, took me about 10 hours in the driveway, but only because i didn't have some tools, and because the tensioner bolt was busted off into the front plate of the engine.