i can legally, as per NY state, carry a kobelco sk80 excavator which weighs 22400 lbs, plus the bucket or hammer on our single axle peterbilt or sterling rollback trucks. a d5 weighs less than that, and therefore a d3 weighs considerably less. these are full-size heavy duty trucks, not f-450/f-550 sized trucks, and not something that normally would be used to carry broken down cars or trucks. these are much more heavy duty.
during the summer i drive an f-550 service truck with a 6spd, and i would never even think of putting a d3 behind it. the truck can barley move or stop itself, much less a 18k dozer on a trailer behind it. it would be a disaster waiting to happen. nor would i ever attempt to pull a d3 w/ 4500 or 5500 series dodge or GM. i love my cummins powered dodges, but know the dodge part of the truck is not up to that task. i find it rediculous what some people think they can tow with their big bad powerstroke or duramax. its scary. i work at a heavy equipment rental yard, and people come in wanting to put 8000lb mini-excavators in the bed of an f-350, or tow a 10000lb tracked skidsteer with a chevy trailblazer on a single axle landscape trailer.
BTW, my main job is to drive a tractor trailer and deliver and move the heavy equipment my employer owns. i can legally weigh 175000lbs, so i do have some experiance with towing equipment