I agree with the above posts. Your cockerels apparently haven't reached breeding age yet (but at 3-4 months, it could happen very soon), so they haven't started gang-breeding your pullet, or fighting over breeding rights. These things will very likely happen. Either move her out with one of the boys, or put all the cockerels in a bachelor pen. As stated above, even one pullet with one cockerel will be hard on her. Three of them - she will never get any rest.
While each chicken has their own personality, just like every person, they still have basic instincts. For cockerels, that instinct is to BREED, BREED, BREED - especially when the hormones hit. Just like teenage boys, they don't think of anything else.