The three hands are scored and compared as regular poker hands. The bottom and middle are regular five-card hands. The top only contains three cards, but is scored the same way, thus the best possible hand on top would be three-of-a-kind, while most top hands are high-card hands.
Since each player need 13 cards from the deck, OFC is played with a maximum of four players. Most commonly, the game is played heads-up between just two opponents.