Can anyone confirm that Descent 3 works under Windows 8.1 on a 64 Bit system?

I would assume it would. You can check this thread on the DescentBB
http://www.descentbb.net/viewtopic.php?f=8&t=16700

If nothing is there, ask.

It runs for me (also on 8.1, 64bit) just fine, but not perfectly at first. Who knows, you may have no problems at all, but I had a wonky mouse when first trying the game (which if you have this experience, checking the box "enable logitech mouseman mode" in Descent 3 Setup/Controls tab cleared this problem up). Other than that, just the standard graphics tweaking that you could expect to have to do with any older game, which of course, is optional.