As I said, I might only be thinking about three encounters because it's a cultural pattern.
But I can make an actual gameplay argument in favour of it: The first ambush is you and your partner teaming up against a single enemy. The second ambush is you and your partner against two enemies - one each. It's still easily handleable. A third ambush would be a further step up in difficulty: You and your partner against three enemies, which would make the third encounter further challenging because now you may not have enough manpower to engage all the enemies at once. You would have to think about the third fight a little more, a suitable capstone. It's not really important at all, though.
