Hi Reggie,
I have never used a 3ch radio set myself, but I guess the throttle and the wheel just take the form of two separate channels on the receiver. If so, you need to connect one servo to the throttle channel and the other to the wheel channel, and then enable some sort of V-tail mixing, be it using a plug in mixer, or on the transmitter (if the dx3 has V-tail mixing built in) This will mean that the throttle will control both servos for forward/backward movement, but the turning of the robot (reversing one or other servo) will be done by the wheel, ending up with control similar to an RC car on the same transmitter.
Hope this helps, although I may be spouting rubbish,
Alex