How do I add a 3d model to FCPX

I don't see any 3d formats listed in the official media import docs. Media formats supported in Final Cut Pro - Apple Support

I've never tried any, but I see there are plugins that say they can import 3d models into FCP like mObject and Eneo3D.

You mean and manipulate it in FCP? You need to do that in Motion.

Not directly.