Boris keep in mind ST3 is not an elaborate mixer it actually gives you 6 stereo tracks that can be tweaked and 2 stereo outs, surely this is enough one full stereo as main mix to all members and one separate click track to dummer, now this goes to your mixer , now this is where you should be creating your individual in ear mix of microphones and instruments and the stereo backing tracks for every performer, and if the drummer needs a special click track you have the other stereo out to supply a special need client, you have to remember ST3 is just a backing device just stereo output is great not to mention you can seperate in to 6 sections of backing in stereo Drums , guitars, Bass, keyboards , solos, click
Normally people don’t need drums as it’s live , or guitars as they are live so I can’t see a great need for other devices being involved when one device does so much.