Usually you just put a few dashes of bitters in cocktails since they are very strongly flavored - 3 bitters would usually mean three dashes/drops, and then you can always add more to taste!
Larkin M. asked, “What does this taste like? What do you use it for? What do recipes mean when they say things like "three bitters"? And what do you like it mixed with best? Sorry 4 questions in one just dont know anything about this!”