In Qlab3 Settings Setup MIDI controls to Use Musical MIDI. I am using ch16
Screenshot QLabMID Controls.jpg
Create a Mainstage Patch and put a drum pad in the layout. Assign the hardware input to something ( if you are not using hardware inputs assign it to an unused IAC bus)
Screenshot MainStage 3Button Creation.jpg
In Mainstage 3 Edit
Add an external instrument channel strip
Name it Qlab
Route it to Bus 1 IAC CH16
Route Input and output to None
(You could have Qlab send 2 channels of audio into this by creating a cabled loopback if you wanted to do some crazy stuff like vocode Qlab audio cues in MainStage)
Screenshot MainStage 3 Channel Strip.jpg
In Mainstage 3 Edit
Select drum pad
Assign Qlab Midi Notes C-2 to pad
Screenshot MainStage 3 Assign.jpg
In Mainstage edit Assignments
Turn off send unassigned MIDI to all channel strips
(This is probably what is causing your Qlab MIDI trigger captures to fail)
Screenshot MainStage 3 Unassigned MIDI.jpg
In Qlab 3
Create a cue
Switch MIDI Trigger On
Click Capture
In Mainstage Perform
Click drum pad
Trigger is captured
Enter 'any' into velocity window
Screenshot QLab MIDI Trigger.jpg
That should then work.
Mic