This thread is old, but I'm replying it in case it can help users with this same question in the future.
Having been using AutoCAD since release 10 (back in the early 90s), I couldn't believe the conversion from 3D solids to 3DFaces had been removed from AutoCAD. So, after digging for a couple of hours, I found how to do it with current AutoCAD versions (no other tools needed). And, by the way, it's rather disappointing that it was so hard to find how to do this (maybe nobody needs to export AutoCAD solids to raytracers that don't support DWG, or what?).
The procedure:
1- Set FACETERSMOOTHLEV variable to 0 (if it's not zero, you'll get a subdivision surface, which you obviously don't want when you are converting a solid into 3DFaces).
2- Set FACETERDEVSURFACE and FACETERDEVNORMAL as desired, depending on the accuracy you want for discretizing curved surfaces into 3DFaces.
3- Execute MESHSMOOTH on the solid you want to convert to 3DFaces
4- Explode the resulting mesh (surprisingly, you directly get 3DFaces instead of a Polyface Mesh)
5- If your purpose was exporting the geometry into external software, export the 3DFaces as a DXF for example.