If I understand your intent, you plan to put the back wheel of the trike on the rollers but not the front ones? I'm not sure that will work spatially - you might have to turn the trike around, which would take up a lot of floor space. Also, the rear wheel drives the front in a typical roller setup. This drive mechanism involves some work and friction. Without it, I'm afraid you'll be free-spinning without getting much of a workout. All I can say is try it and see. Like others said, there's no reason the 3-speed won't work as long as it fits the rollers well and the tires are reasonably smooth.