Being one who learned to fly on 3-channel, and who currently flies 2 and 3-channel, there is nothing wrong with learning on it if that's what you choose to do. The jump from 3 to 4 channel is not that much, more of learning to use your left stick to steer on the ground with 4-channel vs. using the right stick to steer on the ground with a 3-channel.

That being said, you're better off getting a 4-channel plane to start with. DO NOT disable the ailerons on a 4-channel plane to mimic a 3-channel unless you want to get into trouble really fast. I doubt the cost difference is that much between a 3 and 4 channel, and when you get a radio it'll most likely be 4 channels or more which will have 4 servos in it, so just get the 4-channel.

Hogflyer