We really need to have 3 more Mistborn than the defenders? I think the rule is rather vague on that point, and think about it from an in-world perspective. If we send 3 Misborn, and they defend with 2, one Mistborn still got through the defense, and that’s enough to kill someone easily. I interpreted the rule as you need 3 to start an assassination, but if a single one gets through it is successful. Otherwise you could have the attackers send 3 Mistborn, but then be fended off by a single one, and that doesn’t seem faithful to the idea of a house war.
We could use the rules of Risk, this is actually almost the same exact thing. The attackers get to win if even a single Mistborn gets through, but the Defenders have an advantage in that they win if there is a tie.