@bakerja4 yes it makes perfect sense and that is the issue right there. In radio what you need is called a "mix minus" which means the entire mix "minus" what you need to take off the bus, namely the caller on the phone.
Keep in mind that you quoted me talking about monitoring sources
in OBS and not on hardware, but the principle is the same.
In your case you need to somehow get the "caller" that is, the PC audio off the signal going out to the live stream. I would put the mic on a bus and send only that bus out to the live stream. You monitor off the normal Main stereo bus but the stream only hears that aux bus with the mic on it.
How do you do that on a Model 12? I have no earthly idea. Is there a way to listen to the PC without it going to the Main bus? Maybe PFL with the faders down?
You may have to use ASIO instead of Windows audio in order to get access to the individual outs on the console. If there is a direct out for channel 3, that is what I would use and then only the mic is feeding the live stream. But you are going to have to use the setup on the live stream site and see what is available for audio sources from the selector. Some software won't use ASIO and is limited to Windows audio only. I have yet to see a web based broadcasting interface that would utilize AISO audio.
Believe it or not, even with a fully functional studio with
two consoles I use a
USB microphone to both send audio and also monitor audio right off the microphone with my headphones plugged into the monitor jack on the microphone. When I do this, the Behringer X32 is sitting there switched off, with the cover on it, holding my papers as a desk, lol. But it's the most practical and easy way of doing it.
In this video I'm testing the JTS JS 1-P on a YouTube live stream with PRISM Live Studio PC. Monitoring right off the mic. That is
by far the easiest solution to your problem.