No, I mean the three countries.
Hmmmm... now you've piqued my interest. I love trivia, and this sounds like an interesting trivia question, if it's legit.
If *indeed* there could be said to be a third country bordering the USA, I'd guess Russia (after Mexico & Canada, of course).
My reasoning? Their "national waters" border ours at/near the Diomede islands.
Am I close? Lol!