Like Rocky said, make sure you plenty high in the air.
While going full throttle gently pull up with the elevator until the nose pointed upward just a little bit.
The apply full rudder while releasing the elevator. When the plane is almost up side down, apply down elevator while continuing to hold full rudder until the plane is right side up again.
It's really all about timing and can take some practice. Not all airplanes can pull this off. Sometimes trainers can be too stable and have too small of control surfaces to be able to roll without ailerons.
Hope this helps...
Channel Roll by: Rocky
A roll with a three channel roll is very hard to do, make sure you have lots of altitude, and right before you go inverted, come in with a little bit of down elevator to hold the nose up, and then release it as you come into the other knife edge. Also, make sure that you have plenty of altitude, as that is how I lost my Sig Kadet Senior kit.