What are the 3 wheels on a roller coaster?

Road Wheels-these run on top of the track.

Friction Wheels-these either run on the inside or outside of the track, depending on the builder. They are used to make sure the car doesn't go off the side.

Upstop Wheels- these are on the bottom of the track so that when the car gets airtime or goes through an inversion, the car stays on the track.