First off, the odds two suited cards will make a flush is 15 to 1

In a 10-handed HoldEm game, the odds of someone else being dealt two suited cards of the same suit as two suited cards you hold are 3%. Your chance of being the smaller flush obvious depends on which cards you are holding and what cards appear on the board. Typically you don't need to worry about playing smaller flushes, but you should take note of the action. If the 3rd flush card comes on the board and you still get heavy action, you need to asses your situation--will the opponent play aggressively with a smaller flush? will the player still push his two pair/set/top pair this hard on a 3-flush board?