This can be a tough thing to negotiate and falls under a larger umbrella I’ll call: The Ways and Means of Shared Living.
You didn’t state your age, but let me start by saying this: part of your twenties is figuring out how you operate and what your threshold is in shared living spaces. It’s a time to test your cleanliness against others, see where you stand on replacing the toilet paper roll when no one is looking. What kind of roommate and human are you? Do you empty the garbage or place another dirty napkin on the bulging mound of trash? Do you share your time, and space? We’re all socialists until someone takes the last rice pudding and you find yourself screaming into the open fridge: “Seriously guys, who the hell took my Kozy Shack Cinnamon Raisin Rice Pudding?”
When it comes to your plus one sleeping at the shared house, there is no set rule. It depends on the nature of your roommates and the nature of the house dynamic, both architecturally and socially. It sounds like you guys are openly communicating about this issue, expressing your needs, grievances, etc. That is the key to all of this. Three days is, as you know, almost 50% of the time. It’s not NOTHING, but the other side of the argument is: Screw her. She’s not your mom. You can have your man over as much as you want. You’re an adult paying rent, damn it!
I will also point out that you refer to her as your roommate, not your friend. If in fact you are just shared habitat dwellers then be respectful while doing what suits you. If it’s untenable then in a few months one of you can bounce. But, if you are friends, or have a long history, it’s worth exploring why it pisses her off. Does she feel excluded? Does she miss you? Is your BF being a room-dominating dick? Maybe it’s just a matter of correcting or addressing a few things and it will be all good: a bit less Melrose Place and more…Full House.