for my levels i use a combination of programs.

1. make level in sketchup pro (taking care to make it all convex)
2. export to .3ds file format
3. downloaded and installed Gmax
4 downlaoded and installed Tempest (quake III plugin for Gmax)
5. load tempest and select all the level
6. right click and convert to editable brush
7. export to .map
8. import into 3dgs and scale it to correct size

i havnt yet figured out how to export to 3ds import using tempest and export to map with all textures.

another alternative is icex 2.0 which was made for 3dgs but i havnt bought it