EDIT: You stated in comments that your landlord is OK with the bf over all the time & knows about the roommate.
With that new information - If utilities are included in the rent then the bf coming over all the time has no effect what so ever on the complaining roommate. This twit has no right to object to rules agreed to by the landlord. Landlord says bf are ok then she gets no say in the matter. If she does not like it she can move.
Your third roommate rents a room. She can stuff it.
Here is the agreement: she pays her rent, or she can get out. Her rent stays what it is, or she can get out.
"My roommate and I" ....
Has the third roommate been approved for residency by your LL and has she been added to your lease? If the answer to both questions is "No," you have bigger problems than just talking to her about the amount of rent she pays.
If she is not there with the approval of your LL, all of you are subject to eviction for a lease violation.
Read your lease when it comes to the amount of time guests can be there. If your boyfriends have exceeded that amount of time, they either cannot come over any more or they also must have the LL's approval & be added to the lease.
The third roommate is certainly within her rights to complain about your boyfriends being there constantly, especially since they are not paying rent and most likely nothing toward the cost of utilities.
You had better check your lease on guests. Surely the boyfriends are using some utilities, water/electricity etc. If your LL finds out about the constant guests, you just might be evicted. The noise, you need to make sure the boyfriends respect quiet time before this escalate and go around your LL. You are overstepping the boundaries by having boyfriends over. There should not be guests over this often Careful if LL finds out.
No noise after 10pm is standard anywhere. Maybe you are louder than you think. Read your lease and you may find a provision that limits how many nights guests can stay. Most likely around 14 days per year or such. Your roommate did not sign up for a house of 5 people just 3.
Perhaps your new roommate was under the impression that the majority of the time the rental unit would be occupied with only girls and she feel uncomfortable with your boyfriends over so much.
She offered a possible solution, however, this might be for awhile after which there would be other problems she would find and want to address.
The other points mentioned by other posters are valid in the increase in the usage of utilities, thus causing the utility debts to be higher. The water, gas and electric bills might not be as high if your boyfriends were not there as much and using these utilities.
I hope this has been of some benefit to you, good luck.
"FIGHT ON"
if the boyfriends are using utilities, shared bathroom, etc it does affect her, so give her a small break on her share of the rent, offer her a discount of 10% and you and your other roommate split it, or else get a new room mate
As long you are being reasonable
The bfs may not have a key but if they are there that often they are using water and other utilities so your roommate does have a point.
The bottom line though is that, as she is the 3rd person, if she doesn't like the living arrangements she should find somewhere else.
As long you are being reasonable, she will agreed to your agreement.
Although your boyfriends doesn't hang around the house but the fact is 5 people living inside the house. In addition, under some residential house lease agreement, some house owners doesn't allow more than x people living inside the house.
Me and my roommate (we have known each other for a year now and have lived together at the same time) have a third roommate who is bothered by our noise and boyfriends. First off, me and my roommate leased a three bedroom house together and rented the third to a student a month later. Third roommate wants us to be quiet after 10pm. But now she complained that our boyfriends are over too often and wants them to pay rent. She is bothered by them because they add noise during the night. Our boyfriends are respectful and don't bother her other than saying hi when they meet each other. They usually just stay in our rooms and not hang around other places in the house. And yes they are at the house quite often but they don't have keys and are there when we're there. She gave us a compromise to either have them over 1-2x a week and she'll pay her rent or lower her rent and they can be over as much as they want. I need help on what to talk to her to have all of us in an agreement