Act 3 brings synergy to your story. It’s where all your hard work in teaching your main character to learn and grow pays off, and now your MC needs to prove just how they’ve changed.
That growth, or the lack of it, is determined by the kind of story you’re writing.
If you’re writing a romance, the MC will realize that the other person is exactly the right person for them.
If you’re writing a fantasy or adventure, your MC will realize their own strength or full potential.
If you’re writing a tragedy, your MC might not learn their lesson at all, and fail to reach his goal.