Thanks for the help, I haven't actually got a Helix yet so can't try it out. I've been extremely tempted and the ability to do this might tip me over the edge!
The bassist uses active pickups so it sounds like he could go into the AUX IN, then the second guitar could go into one of the FX returns? (or possibly just have them both going into returns).
I'll then be using the XLR OUT to go into a mixing desk, this means the 3 guitars will all be on one channel on the desk so I wouldn't be able to balance the volume levels from there. I'm assuming I'd be able to alter the levels of the 3 signal chains from within the Helix?