Not to confuse the issue:
You said in your original post you are considering an electric glider. This means it has an electric motor (mostly used just to get altitude) and that means one of the channels will be to control the motor.
any additional channels would be flight surfaces. Yes I too agree 4 channel is better than 3.
To go a step farther, a computer transmitter with at least 6 channels would be recommended (even for a 3 channel plane) the transmitter would serve you well on many other models in the future.