It looks like there might be an issue with plug-in detection of channel 3.
This might be caused by:
– You need to use a mono-cable. Otherwise plug-in detection does not work. Make sure you are using a mono, not stereo cable
– an actual hardware issue. We have prepared a firmware version that shows the state of the plug-in detection (soon to be a feature). please get in touch via mail and we’ll send it to you for debugging.