RE: will 40 size trainer fly on three channels
Most trainers do OK with the ailerons locked and stock dihedral for Rudder, elevator, throttle mode.
Rudder at slow speed can be more effective than ailerons.
Just for an experiment... take off normally with your common 4-ch trainer and fly it a while at altitude not touching the ailerons. You may be suprized at what you can do. (its a good thing to practice in case the aileron extension comes unplugged in flight...)