I can’t imagine it being all that difficult.
Create a map where the third countries bases are scattered evenly amongst the other 2 and disable flight for that country.
How does that affect ENY?
How does that affect auto-switch?
How does that affect the win-the-war field ownership percentage?
What happens on a reset and the countries rotate position?
Setting it up is one thing. How it functions is another. The problem is that behavior is undefined (because you are circumventing the designed behavior) and the negative effects are hard to predict and HT doesn't like to "experiment" with the Melee. And no other arena is allowed to have the win-the-war logic to test in.
But even if it were technically feasible, HT has stated he won't consider it.