They are not yet sexually mature. Once those male hormones kick in there is a good chance that pullet is going to be terrorised and mated half to death by them. There are circumstances where you might be able to keep a mature hen and rooster together with two adolescent males if they had plenty of room, but 3 adolescent males and one young pullet is not a healthy or happy combination.
Males usually mature sexually before the females and are at the whim of their hormones and all they want to do is mate. The pullet will almost certainly get forcibly mated by all of them repeatedly. In a more natural situation where they were raised in a family group with mature hens and a dominant rooster, he would protect her from the sexual advances of other males until she was mature enough for him to mate and the older hens would put the young cockerels in their place and teach them some manners. If these birds are all the same age and being brought up together, there are no mature birds to keep them in check and it can get ugly very quickly.
I would urge you to get rid of or make a bachelor pad for the boys and find your pullet a couple of female friends. Even with just one cockerel she will most likely be overmated and end up with bald and possible raw patches on her shoulders and neck. There is a big difference between a mature rooster and an adolescent cockerel that many people fail to appreciate.