For this kind of behavior, my method is very simple, just say it directly. If persuasion fails, don't save face for him, let alone fantasize about being his classmate. In this case, it is recommended to respond directly to the counselor. If this doesn't work, then respond to the leadership of the college and change the dormitory for the man. Dormitory is a public place for everyone. We can't let some people do whatever they want. I believe the school leaders will take care of it.
The other is that either you let him move out or the three of you move out. It is wisest to focus on the useful places. If you don't want to, you can only be less particular than them. For example, you three wear less clothes and stay at home. After all, you also took the room fee. It's okay to wear less or even naked on your own turf. In this way, boys will also reflect and take their girlfriends out to live.
Or you can tell him that his girlfriend is very beautiful while eating and drinking in your dormitory. This method stimulates his sense of crisis, and he doesn't want his girlfriend to stay in your dormitory and go out to live! This will not affect the feelings between your roommates, who have gone out to live.
You can also tell this boy's friend to ask his best friend to tell him that you don't like it, and then he will silently reflect on whether he really has a bad influence and rarely brings his girlfriend back to the dormitory. After all, it is really inconvenient for a girl to live in a boy's dormitory, and there is no privacy.