You do not have to worry about setting this -- your coach will automatically set it at the end of the game for you. More technically -- your available players will be ordered by scoring odds. So each round, you are always shooting your best available offensive player.
A player's scoring odds are determined in the simulator by looking at his real life stats and his current, in-game fatigue level. All 20 players on your game roster are eligible to shoot unless they are injured or have received a game misconduct (ejection) during the game.
In the event that the shootout remains tied after the 3 required rounds, 1 additional round will continue to be be added until a winner is determined. In the rare event that it happens to remain tied after all of your players have had a turn, the order returns back to the top of the list and starts over with the best player. All available players on your game roster must take a turn before any player can have another turn.