I recently needed to refine a few synth patches and decided to use my Kore 2 controller and map the most important parameters in mainstage 3. I also decided to create a template as I need to see what I'm doing. So in case someone still uses the Kore controller in combination with Mainstage 3 I'll share my template with you Have fun!
Nothing, you can map the knobs however you want - patch/set/concert specific to every parameter mainstage offers. Basically this contains just the knob/button arrangement and a kore-background graphic.
it goes in a folder named "MainStage Grouped Controls" which should be in your logic/mainstage config folder. For me this is in my music folder, but I changed this somewhen back in the days and now can't find where the default path should be. So the easiest way is to create some "dummy" grouped control yourself, save it with a unique name and search for it in spotlight. Then put this file in the same folder