If you are talking about "audio>A3", that setting enables you to change track 3. Then after setting that, you need to change the camera angle in the multicam window. Then the audio track will change.
I just tried it on track 2, and it seems to mess around with something in FCP- the only thing I can change after that operation is track 2. I can't change track 1 after I have changed track 2, no matter what I set. Probably a bug, or my project may be corrupt.
Anyway, having different audio tracks (split mic-ing) on different angles and switching between audio on different cameras is a bit of a pain, unless you are switching both the cameras and the audio. You also can't see what you are doing.
www.strypesinpost.com