When I was looking to tow a trailer with motorcycle behind a car attached to the motor coach, I was informed, by print from the manufacturer, that it was considered triple towing. I don't know how they arrived at that decision, other than perhaps the "local" law, but I decided it wasn't for me. I think the laws are too open to local interpretation which can lead to problems down the road.
Somewhere I read that in CA the last unit must be a boat. Not sure if that's true since I have not read the CA motor vehicle rules.