Thanks for your reply. I took some screenshots from my actual MainStage interface and the MidiGuitar 3 interface from one of my patches. Wrote some annotations of how I use it, I’m new in this so I don’t know if I’m using it appropriately but it works with MG2 and the older MG3 0.48 beta 
Don’t have any problems using MidiGuitar 3 standalone.
Thanks a lot for your help! 
The Audio Unit screenshot.
I must add that I updated to the latest beta and still have the same problem.
There’s no clear reason for it not to work. I built a preset based on the limited information in your screenshots and Analog Lab V received MIDI just fine from MG3.
Have you tried duplicating the preset again? Can you succesfully select a different MIDI source to verify that MG3 is causing the issue?
FWIW, I’ve tended to use MIDI Guitar 2/3 at the set level rather than with an instance per preset. I’m not really familiar with how things might respond in your per-preset configuration.
EDIT: Actually, I had an idea and came back to your problem. Duplicating presets creates some issue where both MG2/3 are present but I think that’s more about Mainstage’s clunky MIDI routing than MG2/3 themselves. Maybe you should try using a set level instance, or use MG2/3 as a standalone and send MIDI to MainStage. I vaugely recall this coming up on the forum here a long time ago mentioned by member Paul Driessen, I think. You could probbaly search it up.
Ok, thanks, I’ll give it a look. Honestly I didn’t know that you could use the standalone app and connect it to MainStage hahahaha, I have a lot to learn, I seriously need to find a tutorial to sort out all of this. I use the patch/instance approach as that’s how I’ve been managing my guitar effect plug ins so I thought of using MG3 the same way. I just find weird that latest MG3 betas refuse to work with my approach as MG2 and 0.48 used to do without problem. Fortunately I just kind of found a workaround just loading Arturia/other vsts inside of each MG3 instance and removing MIDI output from the chain so it kinda skips the “MG sending output/Instrument receiving output” step which I think it’s the problem here. I’ll consider my problem solved just now but I’m not satisfied so I’ll continue looking for a cause and I’ll try to learn how to use the standalone app with MS as I don’t know what having multiple individual instances would do to performance in my machine. Thanks a lot for your help Vaultnaemsae.