I enjoyed reading your story, well written and relatable! In our case, we're an all-female triad with a 9-year-old son (biologically mine). We stick to the thought of us being one family unit and that's what strengthens us.
I always say it's freedom, honesty and respect that keeps us together. Everyone is free to love and be loved by someone, yet for some reason we don't venture out there to look for someone. It just feels good to have that confidence knowing that if ever I get attracted to someone, my family won't judge.
My son loves us three, having moms with totally different personalities. Me who's a bit strict, one who spoils him and the other one he could go play ball games with. If there's any confusion in his part, it's that mother's day is coming and he has to be 3x more thoughtful with the gifts!
That being said, we're not perfect. But approaching life with the notion that we are all individuals supporting each other in this Earth journey is what keeps us happy together.