Let's imagine this:
A world wide model airplane manufacturer will sell a new 3 channel trainer (No rudder) with 50% off the normal price of a 4 channel trainer.

Would you buy it?

In my opinion this does not make sense to me, I learned to fly with Rudder and elevator only (2 channels), then appeared the throttle control and finally the ailerons, which were a fancy ad to our models.
Of course, now, things have changed and the use of ailerons is very important in flight, many routines in pattern flight need ailerons. (And all routines require a bit or a bunch of rudder)