.obj and .3ds

Both formats .obj and .3ds are supported by SH3D, just drag-and-drop, also if zipped. As hansmex writes, there is (usually) a .kmz (or .dae) alternative to the .skp download. .kmz is a zipped file that will have to contain a .dae file to work, which it does in the 3D warehouse alternative downloads.

ok

PS For the record, .kmz files can also contain other kinds of files, like .kml, used in Google Earth, see google documentation. Not that it matters, but for the fact that not every .kmz file contains a lovely model...