Internet Gaming Connection Issue that no one can help me with

Hi Guys,

I have been trying to sort out this issue I have for ages now, I hope someone here could possibly shed some light on it.

There are three of us who regularly game on the net.
When playing games like Battlefield Bad company 2 or any other Hosted Server based game we have no issues.

But games like Civ4, Age of Empires 3, Settlers 7 (even though you need to login to their stupid DRM server thingy) are causing us Great pain.

To make this a little easier to explain I am going to Describe the three of us as:
Player 1, Player 2 and Player 3.

Player 1 uses Bigpond as an internet provider
Player 2 uses Bigpond as an internet provider
Player 3 uses IInet as an internet provider

Our problem is this:
Player 1 and 2 can connect and host any game with each other.
Player 1 and 3 can connect and host any game with each other also.
Player 2 and 3 can not connect or host any game with each other.

If Player 1 hosts a game, either Player 2 or 3 can join him, but Both cant (who ever goes in first, stops the other from being able to enter the game).

We have been able to play Civ4 and Age of Empires 3 using Humachi and then connecting using DirectIP connection in the games, but NOT all games have that option (Settlers 7 as an example).
So although this does let us play some games, it doesn't solve our problem.

Now Player 2 and 3 have both changed PC's, changed Modem/Routers, turned off all firewalls both software and hardware. Updated drivers, etc etc.

The only thing that has stayed constant is our ISP Providers, everything else we've updated, changed, etc, STILL NO JOY (small cry).

Besides the theory of a Little Green Goblin messing with us I can't for the life of me figure this out.

Any help would be massively appreciated Cheers