To all new comers, here is what I understant about friendship among girls, it needs about 20 points to go up 1 tier.
Ex: 20 points for Aquaintances to Friends and 20 points for Friends to Good Friends.
Each activity you do with them together like swimming, training, studying and movie night ups 1 point, so you have to grind a little to get them to be friends with each other.
As part of the story, and if you don't mess it up somehow, Rogue and Laura will become friends automaticaly after the Laura event where she asks you to be her friend.
Basically, get the 3 girls together and all kinds of things like swimming, studying, trainning and movie nights and if you can manage, also try to bring them to the BBQs and Pool Parties.
Also, you only need to have them 3 in the Friends tier so that everyone accepts the poly, so you have to only grind 20 actvities with all three of them together basicaly.
Hope that helps.