[Imp]

If you really wanted a 3D viewer the easiest way as said would be to use an old free 3D game engine like the real one. More than up to the job & does LOS but tend to be based on square grids not hexagonal ones. Might find one if check out some RPG games they are set up with basic light sources & to check LOS between 2 selected points. Can control visibility range to but once you have done that though you may as well produce some units to shoot at each other.
You need programing skills for the above & checking LOS is one of the trickier bits, need to set above ground level or the slightest thing will block it.

The other option is to use a proper 3D modeler like lightwave, sure there are free ones not lightwave though thats a lot of notes. Or a 3D terrain generator there are definetly free ones.
All will easily be powerfull enough & can read bitmaps so if you fidled about could probably produce what you want but would probably want a bit of programing to do it that interprits map data.
Otherwise time consuming as need to produce 2 pictures bitmap & bumpmap for terrain type & heights which will require an art package to mod pallettes.
Problems
With 3D terrain generator & possibly basic (free) 3D programs are light source first case is assuming the sun, probably cant adjust acuratly enough. Also placing viewpoints accuratly for LOS could be a problem if you cant snap it to a point.
Realisticly you are trying to produce a landscape of several Km using a few points. I would probably use more building a kettle in 3D so the results will look terrible go back to game engine idea now I have thought it through.