There seems to be an API for M3: https://m3ideas.org/2012/02/23/how-to-call-m3-api-with-rest-json/
So that would be the approach to get data out of that system. As far as I know there are not yet made any standard connectors for that API. But REST calls ara easy from Mendix so you should have that working pretty soon.
Regards,
Ronald