Another possibility is can you use autocad? The coreldraw files can be saved as dxf files, although intitially they will by in just two dimensions but presumably it would be then easy enough to give each level a z as long as they are seperate sets of data and the depth can be identified (which is not always possible on many old plans). I can produce dxf files but can't open them as I don't have autocad.
It may also be possible that ways of doing this have already been done in the caving world. In Matienzo with over 300km of surveyed passage now the survex data has been imported into 3d digital surface mapping in autocad, & I think footleg has been working on 3d presentations, but I havn't really got into what he has done, more interested in pushing the caves for real.