Frankie
Your application of the PAS is going to be a bit more complex than a OMB situation. Of the 4 inputs, only 2 are adjustable by presets. With 3 vocalists, you will need a mixer and have to find one preset that suits all their voices the best. Same goes for the mulitple instruments you want to run through it.

Channels 3 & 4 are "flat" (not customizable by presets) and best used for pre-mixed, pre-processed, pre-recorded music such as mp3s and CDs.

Eddie