It would be the one of the three that has the highest flush suit card in their hole cards.
For example, say the board is Jc, 8c, 2c, Kh, 9s. Person 1 has 7c 6c. Person 2 has 10c 3c. Person 3 has 5c 4c. Person 2 wins because he has a flush and his 10c is the highest flush suit card in play in a player's hand.