If you own a copy of the game, I *HIGHLY* recommend getting the Dateiliste installation ISO and installing from that. That will give you everything you need and more, including Vortex, D3Server3, a whole bunch of maps, and more.
Aside from that, it's just a straight install from the CD's. I recommend running in administrator AND for compatibility for Windows XP Service Pack 3. Run the launcher and ensure your video and audio settings are setup, you'll want to detect for OpenGL.
As for playing... Peer to Peer is really bad in Descent 3, you really need a 3rd person to host a client/server game and observe, or play on a dedicated server. Foil hosts the Descent Retirement Home servers and has been very responsive to a few of us with level requests.
As for active DCL players, you've got me, Lady Silver, dues, Zero, and sometimes Behemoth and Bahamut. Entropy has threatened starting playing, too. Souljah and Rethink have also played in recent weeks.