There are only 2 sets of tires (in 3 resolutions). A left side and a right side. Note that any custom tire you place on the front will also appear on the back (front and back share the same models).
This is why you never see any vehicles in MTM2 with larger rear tires.
Placing one front tire on top of the truck may make it flip over, or at least handle strangely. To make a drivable 3-wheeler, set the X-axis parameter for both front tires to zero in the trk file.
You might be able to complete your idea by placing an animated texture model on top instead, although that means the "circular saw" wheel on top will always be rotating, even when the vehicle is not moving.
Here's a few examples of what I am talking about. Feel free to dissamble them to see how they were made.
(Rotating blades)
http://mtm2.com/~bigdogge/trucks_page3/slasher1.zip
(rotating circular saws on "I SAW U"
http://mtm2.com/~bigdogge/trucks_pageset/battlebots.zip