We have the 2 motors for controlling the wheels, but need to add an additional one.
Where would we plug in the wires on the bot and what would I have to change in the Motors Setup in RobotC to get the motor to work?
Thanks.

Assuming that you are using the 12V Tetrix motors then you would need to have a second HiTechnic Motor Controller which you would daisy chain to your first motor controller (or to the servo controller if you are using it). Then you would attach your third motor to either motor port on that second motor controller and tell the Motor and Sensor Setup wizard exactly how you hooked everything up.