For sure, we support the Majestics in all versions independet of which sim-version you use. It was only a little bit strange for me and therefore the question. I don´t have P3Dv3 installed so, I can´t really reproduce it …
So, first of all please post a screenshot of your settings page in the FMS Data Manager to look if your P3D root path is correct so far:
Further a screenshot of your Majestic Addon Mapping - move your mouse over the path in the Addon-Mapping page that you see the whole path
… and last, please upload the screenshots directly here in the forum and not somewhere external via link. A description, how you can do this - do you find here
Thank you,
Richard