Surprised they're not just creating 2-4 archetypes and assigning girls to them.
They deliberately didn't go that route.
They use the"Girl" class, which contains the methods for setting up love and trust, as well as most of the framework for how they operate. Instead, they made individual profiles for each girl. It wouldn't have saved any significant time with only three girls since they would have had to build the archetype profiles.
By taking a little extra time and doing it this way, the individual girl profiles help to guide how all of the interactions evolve. There are thresholds for everything from simply talking late (as you probably notice early in the game) all the way through the various sex acts. Most of these are coupled not only with those score values, but other conditions as well.
Even if you cheated in having Jean at 10,000 Love / 10,000 trust, you aren't going to be able to get Jean to fool around until other relationship flags are triggered, which are done as a part of relationship events.
This profile based structure also makes it so that the girls inherently dress differently, not simply because they have different clothing sprites, but because the values for how they prioritize covering different body parts (shame) interacts with respective garment values.
TL;DR - They put a lot of effort into setting up the framework to let them expand without feeling stale.
