If there's a hole one of them needs to fit into, and it's very small, then the little one does it.
If they need weight to hold something down, then the big one does it.
There's probably never a situation in which only the Medium guy would be best to handle it, but that might be funny. Maybe he's troubled mentally by feelings of inadequacy and worthlessness because of it.
In a story like this you would assume in the end, some major catastrophe would come up meaning death for everyone, and ONLY the Medium guy could handle it, thus proving to himself he is worthy.
I think in my story though, I'd rather have that situation come up and he attempts to take care of it, but he realizes he's TOO medium-sized to do any good. Then everyone dies. If he were either just a tad bigger, or smaller, he could have had success.
After this happens the Medium guy is so distraught he decides to kill himself. He tries to chop his head off by standing under spinning airplane blades but he's too short to reach. He tries to jump out a window and he's too big to fit through. He tries to use a gun but he's too little to lift it. He finds out he has too much body mass for the bottle of poison he drank to kill him. His neck is too small for a noose. His head is too big to fit into an electrical socket. etc...
Then he decides to give up on the suicide and go ahead and live. As he decides this a bundle of newspapers is thrown from a truck, hitting him in the back of the neck and killing him. The headline on the newspapers says GUY NEEDED TO STEER ROCKET AND SAVE THE PLANET! MEDIUM-SIZED GUY REQUIRED.