Here's our setup.  3 players on a LAN, two on Vista, one on XP.  We can play the game just fine as long as only two of us are playing at once, regardless of OS.  Add in the third, and problems start arising.

Problem #1 is the desyncs.  Playing with all 3 of us online, from the same internet connection, pretty much guarantees that the game will desync.  It always seemed to be the last person to join the game that would desync, but that may have just been random "luck".  We were unable to complete a game without a desync occuring (compared to NEVER seeing a desync issue when it was only two of us).

Problem #2 is weird connectivity problems when joining games.  Regardless of who hosts, the first person can join the game fine, but the second forever sits on the "Connecting" screen.  When that person finally exits out, those in the game lobby will see a "Player has disconnected" message, (not their name, it refers to them as "Player"). So it seems some initial connectivity is taking place, it's just not fully getting in.

This problem can be worked around by joining the game simultaneously, at the exact same time.  If we count to three and hit Join, both of us get in just fine.  We just later run into the desync issues I mentioned before.

If there's any additional information I can provide, or if there's a way to send in useful log files for this, please let me know.