I don't know about Jean but I know that it did improve stuff for Rouge+LauraJust found this game, it's great. Can't wait for Storm, although it looks like that's Tentatively February next year? Regarding the girls becoming friends, I barely went to the pool. There was a point where Laura asked a question and I sent her to Rogue and after that I feel like they started hanging out sometimes and becoming friends. i think there was the same for Laura and Jean, but I can't remember. Am i correct that those events started improving their friendships? Thank you!
Their friendship is partly a story thing. It can get improved further by bringing them to the pool together.
All the girls' relationships have different max/min depending on their story. Some girls will never be best friends. The only reason to max out their friendship intentionally (It does increase a bit naturally whenever they spend time together regardless of Null) is for the poly route. A second or third girlfriend requires all people involved to be very friendly and have 600 trust.
If your first girlfriend doesn't approve, do not flirt/date/sleep with any other girls. It will start an event that makes the girls less friendly and avoid you for a few days. get those stats up, talk to the girls, get permission.