When I used MESHSMOOTH I selected small portions of 3D Faces in your drawing, then the conversion went fine. When I tried to select all 3D Faces in your drawing - nothing were converted.
You can also try another method described here:

https://knowledge.autodesk.com/support/autocad/troubleshooting/caas/sfdcarticles/sfdcarticles/Conver...

Maxim