Technical issues with Heroes III.

Hello, I need help with Heroes 3. My three friends and I have Heroes 3 installed from the same disc, meaning we have the same version of the game, but we can only connect in a group of three. More precisely, one guy can't connect with anyone at all. We've already reinstalled the game for him, reinstalled Windows, and disabled the firewall, but nothing helps. When he tries to connect, the game displays a message saying that the IP address cannot be found or the session has been interrupted, or something similar. Please help us figure this out.