how can I add a 3rd dc motor and what codes do I need to put in the program?

Do you have 2 Hitechnic motor controllers?

If so then you need to daisy chain the motor controllers (first one to the NXT, then connect the first to the second)

Then go into the Motor and Servo Setup and tell RobotC how the robot is configured.

Then controlling the 3rd should be exactly like controlling the first two.