I think Peter has answered this in another thread saying that ST3 cannot receive CC commands to control volume, but I thought I would just sanity check my understanding that this is, indeed, the case.
My scenario is that I use ST3 on a laptop as the master session and the band logs into that session. ST3 also controls the DMX for the lighting.
Previously the ST3 output went out through a Focusrite Clarett 8Pre Audio Interface into a Yamaha mixer, which is where we adjusted the backing track volume during live playback.
But now we're taking the Yamaha mixer out of the equation and mixing the vocal, guitars & keyboards in Mainstage using an Akai MIDIMIX. This works really well and has got rid of a LOAD of wiring but we need to adjust the ST3 volume somehow.
I can see how to assign buttons for track volume, but not a fader. Another option is to assign ST3 audio output to 2 of the Clarett channels and then patch them back into inputs and then control as a stereo channel in Mainstage but I think that there must be an easier way.