Newby question on monitoring several channels at same time

[mmb]

Hi, I'm extremely new to recording and trying to setup to record 2 acoustic guitars and vocals simultaneously, using 3 mics: one small condenser for each acoustic and one large condenser for vocals. I'm running the 3 mics through an M-Audio Fast Track Ultra, to use the phantom power for the mics and to eventually record in Garage Band. For rehearsals I am not connecting my Mac, just trying to monitor through a small Behringer acoustic amplifier plugged into the M-Audio Output #1. However I can only hear the mic connected to Input 1. How can I monitor all three mics through the Amp? Even if I buy 2 studio monitors, it seems I can't monitor all three mics unless I have three monitors? How do people monitor several channels with 1 or 2 monitors (or an amplifier as in my case?). Also is there a problem using the Amp as a monitor?
Is there a standard config to play and monitor several channels at the same time using 1 or 2 monitors (or Amp)?
Thanks ahead for any advice.

 

[c7sus]

http://www.m-audio.com/images/global/manuals/071206_FTUltra_UG_EN01.pdf

You need to have the Fast Track connected to your Mac in order to make use of the software mixer. Then you can route your input signals to any output.