The game actually doesn't even allow you to play with 3 players:
(this essentially says that the game is only for 2 or 4 players).
However, the UI allows creating tables for 3 players. The problem is, however, that once the 3rd player tries to join for the game to start - he gets an error. Not a very user intuitive way of dealing with this and I think this would require changing the core code. For 3 players - a bug should be created. Not sure whether there already is. Looking through the bugs quickly - it hasn't been reported yet.