Add Import of 3DM files to FreeCAD
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You need to use the same version and level of Python that FreeCAD is using to install rhino3dm.
You can find out the version FreeCAD is using by using
FreeCAD | About freecad | Libraries
or Looking at the heading in FreeCAD python console
One suggestion
Start FreeCAD
In python console
Select one the the directories listed
If FreeCAD is using Python 3.8
python3.8 -m pip install rhino3dm -t [directory path]
restart FreeCAD
Can be downloaded from https://www.rhino3d.com/download/opennurbs/6/opennurbs6samples