There are only 2 ways to get output on channel 3 of either the LC6i, LC7i or LCQ-1.
Either you send signal IN to the channel 3 input, or you send signal in to the channel 2 input and you have Automode engaged.
If you are sending just 4 channels (Main and rear input) into the converter, you will want to make sure Automode is engaged. To check, you will need to take the top off of the converter and confirm the Automode jumper is set to “enabled”, or “On”.
To check to see where the jumper is located on your converter, please refer to the KB article about jumper locations here.
If you have confirmed Automode is engaged and you DO have output on the channel 2(rear) output of the converter, and you still don’t have output, as a test, take the channel 2(rear) input and connect it to the channel 3/sub input. Do you get signal then? If not, check your connections from the converter to the sub amp and/or the amp and speaker itself.
If you do get signal, we have confirmed that it can output on channel 3, and all you would need to do is jump the channel 2 input to the channel 3 input.
See the diagram below. It shows a simple way to take short speaker wire and jump the signal over to the next input.