If your trainer plane has a tricycle landing gear setup, i strongly recommend using 4 channels and 4 servos. As Crashem said, put the rudder servo to the aileron input (rudder wiggles when right stick wiggles side to side).

The take servo #4 and hook it up to the nose gear for steering. Hook this servo into the rudder port (side to side movement on the left stick)

This way when you graduate to a 4 channel plane, you won't have to relearn about steering the plane on the ground (which will be on your left stick on a 4 channel plane).

Also, it makes it easier on us old, stuck in a rut instructors who are used to steering onthe ground using the left stick. And you will need an instructor to learn to fly (at least eith minimal crash damage).

Take care and let us know how things are going.