Is it possible to use MainStage 3 for my full band to play thru, live?

Absolutely possible. That's exactly what my band does.

*shameless plug*

I actually made a tutorial on this topic recently that may be helpful. If the video doesn't get what you are looking for, just let me know and I'll help however I can. https://www.youtube.com/watch?v=KGwlp1hpl_k

Yeah, in Mainstage terms, each level is like a container of settings, in hierarchical order:

Concert->Set->Patch.

You can do almost anything you want at any level...it just depends on how you use it.

Here's how I use it, and that may help:

1) Concert Level- (An entire gig)
A) Metronome Channel Strip with NO output (so the audience doesn't hear it), with Bus routings to the different Headphones Mixes
B) Reverb Bus
C) All band members headphone Busses going to their respective outputs
2) Set Level- (a complete song)
A) Channel strips for each audio input that will stay the same through the whole song. (i.e Vocals, Guitars, Piano, etc.)
B) A channel strip with the Playback plugin for our backing tracks.
C) Add bus outputs on each of the channel strips going to each band members headphone bus.
3) Patch Level- (Verses, choruses, Bridges, etc.)
A) Lyrics and Chords in a text window
B) Any sounds or instruments that will only play during that section of the song.

Now, if you want to use Mainstage by itself, without automation, you may need to create your channel strips at the patch level for anything that will change often through each song. For example, if you want your guitar sound to change between the verses and choruses...then don't make the guitar track at the Set level...only make it within each patch, so that it will have the different settings.

Of course, you could also automate changes using Logic, but that's a little more advanced. ;-)

Hope that helps a bit.

As Authentic Identity has posted, what you've asked is easily possible. However, be careful with the plugins you use because some add a degree of latency that mightn't be acceptable to some band members.