But what if people come up with some exploit? If someone purposely build around the idea of a 3-person team that will beat all the lower powered opponent?

There exist some teams that will easily beat all player who are lower than them. For example, lightning blitz, all they need is for the chain attack to keep killing and you won't even have a chance to respond, and they have a good chance of killing stuff if the enemy is lower power.

Alternatively, water, sailor, roshi. As it were, it's incredibly difficult to kill anything on such double healer team when you are lower power. So while losing a second initiator will mean they can't do much damage to you, you also can't kill them any better even if they are a character down. And over time, they will grind you out with roshi and sailor growing power over time.

And with the way revive works, it is very easy to "force" a 3 person team at the end.

As it were, any change that would give a "normal" 3 person team, that is to say unfortunate leftovers, a fair chance would mean that purpose-build 3-person team would dominate--because if a purpose build team can't win, how can you expect a normal one to?

So that's why it shouldn't happen, because like many other would-be-nice-to-have such as trade, it have abuse risk, and some players just aren't very nice.