I’ve been *&^%$#@! a bit with installing KiCad (again) today, and after a full re-install of KiCad-nightly the 3D models did not show in the 3D viewer. KiCad defaulted to the “Aaa” path, (which did not work) and after searching for the real location of a 3D file, I changed it to the “Bbb” path, and it worked again.
Aaa: KICAD6_3DMODEL_DIR /usr/share/kicad-nightly/3dmodels/
Bbb: KICAD6_3DMODEL_DIR /usr/lib/kicad-nightly/share/kicad/3dmodels/
The paths above will not work for you, but you can still search for the location of the 3D libraries on your system and then point the environment variable to it.