Polyamory takes a lot of trust both in this game and in real life. You need all persons to be close with you as well as each other.Please tell me you are kidding because I did the same and told all three of the girls that I wanted to date them.
Basic common sense, don't be an ass, don't go behind their backs and don't date the other women until they tell you they're ok with it, and it has to be expressed as such not just a maybe. because a maybe is not consent. there's a quest menu you can look at that, raise the girls trust levels, and have them hang out with you in the pool. that'll make them friends with each other. generally it takes at least two tries at the pool to get the girls to become friends with each other. It's not hard. Just don't be an asshole and get their love and trust levels really high.