Why theme lives between characters, not inside one
What a reader experiences is not a character but the space between two of them, so the same claim carried by three different pairs is what stops a theme sounding like one note.
Why the character who knows the answer belongs early
Your story is a refusal that ends rather than a search that succeeds, so the character who already lives the answer belongs on the page early.
Most protagonists don't need to learn the answer. They already know it. They just won't act on it.
That changes how you write your story. If your protagonist is missing information, the middle is just waiting and the ending is a discovery.
If they're missing the will to act, every scene in the middle is a choice they keep dodging, and the ending is them finally giving in.
The second kind is usually truer. So bring in someone early who's already living the answer. That's what StoryDNA calls The Compass: the person already doing the right thing while your protagonist won't.
In To Kill a Mockingbird, Atticus Finch is defending a man the whole town wants convicted from the first chapter. Scout doesn't uncover a secret at the end. She finally sees what her father was showing her all along.
Real people grow the same way. Most people don't change because they learned something new. They change when they stop fighting something they already knew.
So find the page where your Compass first shows up. If it's late in the draft, you may have written a story about finding out instead of a story about giving in.
Why one right choice at the end doesn't prove your theme
A protagonist's climactic choice on its own is consistent with your story's position without ever establishing it, because a second position would explain the same choice equally well.
How to show your theme without characters preaching
A position can be stated, embodied, or carried by what happens to somebody, and a cast can be fully staffed on the first and starved on the other two.