Audition not picking up 3rd mic

We don't know anything like enough about your system and how it's connected to be able to make any sense of this, I'm afraid. All I can say is that if the mic in question doesn't show up as a recordable source in Audition, then nothing's going to happen. So at the very least we'd need to see most of your hardware settings etc, and what sources are actually available. We don't even know it this is on a Mac or a PC!