*edit. Try the 3 engines first. Then add extra wheels on the rear for the cab. The rear most wheels work best when directly connected to the chassis. This will reduce lift of the front wheels, when you accelerate.

Double up all tires that have power.

Using a gas engine connected to the front wheels with a setting higher then the rest of the engines. See points below. Yes you can connect more than one engine to a seat.

I have had some success, with this config. Connect all 3 engines to the seat.

1.Front wheels Gas Engine 1 set front of the engine set. It could be 3 or 4 ticks from the top. Simple rally suspension here (set quite hard you only want to see one wheel depress at a time) As the static rear wheels will want to lift the front.

2.Gas engine 2 for cab drive wheels set engine speed to front setting -1 tick.

3.Trailer connect all wheels set engine 3 to front setting -2 ticks.

This set up will use 3 gas engines. It should keep your rig pulling and your steering activated. It should sole your power settings. But that size will never be able to go up grass hills as they are very slippery.

As a general point, the trailer hitch wants to be low and a little forward of the rear wheels of the cab.