Your adapter is for stereo - ground, left, and right. You really should use a switched jack. With nothing plugged in, the original mic would be connected. When an external mic is plugged in, the internal one is bypassed. As it is, connect the black wire of the internal mic connection to the shield wire of the cable, then connect the red wire of the internal mic connection to both of the other two wires of the cable.
You can also buy a small panel mount jack to install in the camera housing so that there's no cable hanging out.