Do you want to connect your GoPro Hero3 to the ImmersionRC video sender to transfer audio/video and power the GoPro externally through the Immersion? You can easily build your own cable for this. We are using the Ridax 10-pin mini USB connector with a small integrated PCB together with the ImmersionRC wire option available in the above link (the cable we make here can be ordered ready made from the above link at US$ 19,98 as well, but if you want to save $10 by some easy soldering, follow this guide).
Important note! These instructions are only valid for the Immersion RC video transmitter unless you are positive that your device has the same pin configuration and has a +5V DC regulated power output. Connecting this cable to any other device might burn the devide or your camera. For instructions on building a cable for the HobbyKing/Boscom TS35x transmitters, see our post at http://chargeconverter.com/blog/?p=175
The GoPro outputs mono sound, so sound is only wired to the left channel of the ImmersionRC transmitter. If you want (the same) sound in both right and left channel you can just bridge (short) the left and right channels with each other.