They'll be classified as an MC. My father used to own a trike (not the traditional tricycle) with full-sized rear diff axle and car tires. It had a 1200cc or 1300cc engine, if I'm not mistaken. It had an MC plate.

Even ATVs (quads) are classified as MCs if you opt to register it for public road use. Though, you need to have the proper safety equipment installed, like headlights, tail lights, signal lights, and street-legal tires, among others.