A 3-channel heli can go up/down(throttle), spin left/right(yaw) and go forwards/backwards(elevator)
A 4-channel heli can go up/down(throttle), spin left/right(yaw), go forwards/backwards(elevator) and go side to side or let/right(aileron)
i'm pretty sure you need a 5 channel to do 3d moves (go upside down)