So, we're trying to play Heroes 3 using Tunngle through TCP/IP. The thing is that two of us (me and my brother) play from a local network (1 IP adress, we're using a router). One of us can see a game that the other one creates, but can't log in. And the other one can't even see a created game. We used UPnP - it does not help. Many thanks to the person who can help to resolve this problem.

Update: Looks like we found a solution. I tried Game Ranger and it works okay (at least for now).

Update #2: The game does not work properly with more than 2 players. It just freezes after 2 of 3 or of 4 players make their turn. So, Game Ranger suits only for 2 on 2.