Our robot last year had a three wheel drive system, and there were a few ups and downs from having it:
First of all, we were tippy, and we also had a hard time programming the third wheel, since none of us last year knew C that well.
But, on the other hand, the third wheel made driving the robot significantly easier, since we specially programmed (after we learned to use C a little better
) only one joystick to drive the robot, and to have the wheel turn while we go forward at the same time, putting less stress on the wheels than a common four wheel drive.
A three wheel drive is good, and it is bad, it depends on what you have on the robot (and the experience you have, dont forget about the experience).