I mounted a DaLan 11/4" receiver type on my 89 300CE and have had good experience with it for 2 years towing a sailboat /trailer of ~ 2,000 lb.. While it is a Type 1 hitch, it is rated slightly beyond with a gross weight of 2500 lb and a tongue weight of 200 lb. It attaches with 6-7/16" bolts and backing plates to the unibody channels along the sides of the trunk. It strikes me as reasonably sturdy compared to some other passenger car hitches I've seen. For more info, see http://www.hitch-web.com.
As to a Class 2, there simply may not be enough strength and rigidity in the body structure there to take a 3500 lb load. That's probably why you cannot find one. I think it's pretty rare to get a Class 3 for a passenger car, unless it's a Crown Vic or something that size and construction.