In Illinois, at least, as long as the "scooter" is under 150ccs, all you have to do is have a license, so long as you don't plan on driving it on the road. If you wanna drive it on the road, then all you need to do is get it registered as a Scooter Vehicle, and possibly pay insurance on it, not sure. Again, all of this is based off of how you want it to drive and what not.
As far as I can tell, you can run it the following ways;
Gas Engine driving 2 front wheels, rear wheel steers
Gas Engine driving rear wheel, front wheels steer
Gas Engine driving front wheels, front wheels steer (hard, but would be your best bet, if you plan on making it gas-powered, I think) rear wheel is a freewheel fixed axle.
Gas Engine driving 2 front wheels independently (Tank-like driving mechanics. Would be hard to implement, but might be cool to drive) rear wheel is a freewheel caster (can rotate freely and roll).
For electric motors, you can do the same configurations, but in that case, tank-like steering becomes a rather viable option.
I just had another thought.
You could set it up to drive like a real ghost; where the front wheel set is powered and rotates freely 360 degrees, and the rear wheel is a spring-caster (it freewheels and can turn 360 degrees, but has a preferred direction of travel). In this kind of vehicle, you'd be sliding around like you were actually on a Ghost! It could work with both electric motors and gas engines, too. It'd also mean you wouldn't necessarily need a gear for reverse, since all you need to do is rotate the control wheel 180 degrees, though it might be a good idea to limit the travel to 90 degrees in either direction and give it the reverse gear.
I'll draw up a diagram of what I mean in a bit if it's hard to understand.