There is a baseline parity there. Depending on the choices made during character creation, the ability scores, magic items owned, and other factors, any one of them might be technically and mechanically superior to another. I could easily envision a 10th level fighter that far outperformed the other two if called upon to do so, or it could go the other way. Likewise, depending on actual player skill, any one of them could be played superior to another, with more optimal choices. The level alone merely provides an estimate - actual in-game choice, and the luck of the dice during ability creation also matters.
In most of my games, slight variations of power lever rarely matter as the characters work as a team. And the disparity is never all that great, and it balances out over time as each character contributes to different aspects of the game-play.