I have no experience with the StagePAS 300, but it seems to me it's designed for presentations and/or practice-sessions rather than gigging. Unless it's background music you're after and the only thing you want to amplify is the singer, I don't think it has enough power to deliver a full and rich sound.

When you take a more powerfull system you can have the guitar and bass run through the mixer via fx processors and/or moddelers (like the Vox ToneLab series) or even DI's. This way you can leave those heavy guitar amps and cabinets at home. You'll also have a lot less to consider when setting the soundlevels and changing the total volume should the need arise during the gig.

Choose a mixer with a minimum of 4 aux sends. This way all three of you can have a personalised monitormix and have one aux for the fx-processor.

Take also into consideration that faders on a mixer will work much easier in a live situation than knobs. When you have to manage the mixer yourself during the gig, you can see which fader is in what position and adjust in a moment's notice.

If you want you can post the gear you allready have and a budget you want to stay in. Perhaps we can find some options suitable for your situation. (It's allways nice to go windowshopping in PA-gear-land )

Mike
www.mikewever.nl