Hai provato con questo per il 3D:
You can use a free utility (Dxf2xyz 2.0, downloadable from http://www.guthcad.com/freestuff.htm) to convert from both dwg and dxf files to an xyz file, i.e. a comma delimited text file containing xyz coordinates. Then you need to change the extension file from .xyz to .txt, before you load the xyz file in QGIS by the button Add delimited text layer. Then you save the text layer to shapefile to have the points you need to create the TIN file.