Actually, if you're only using MainAV, you can add up to 3 
more tracks of audio.

Ex Audio 1 and Ex Audio 2 give you the first two tracks.

Now the trick for the third track...

Create a color clip, duration at *least* as long as the audio 
you need to bring in.

Add the color clip on to the Insert AV line.

Apply the Media Importer filter to the color clip, and import 
the audio file you want.

Apply Luminance Key to the Insert AV clip and expand the key 
range over the entire histogram. - OR - Apply Picture-in-Picture 
and move the video off-screen.