@Draconess25 I’ve been in a similar situation. A triad of bi lady, lesbian lady and hetero guy. We didn’t form the relationship(s) simultaneously, if it happened the way you propose it would have been structured differently. You’re wise to avoid marriage or civil union, as that would leave one party “out in the cold” because of bigamy laws. We had to create contracts and other agreements, involving lawyers, to provide as much equality and security as possible for the partner not legally “married”.
We used a combination of legally-binding contracts, wills, health care directives and other written agreements that were not legally binding but all held sacred. We were mutually exclusive sexually, exceptions (rare) by unanimous consent. All major decisions were made in a “family council” setting; some issues decided by majority, some by unanimity. Our goals in this setup were to promote equality, harmony and discourage jealousy.
You should realize that a three way committed relationship is not twice as difficult, more like four times the work as a monogamous pair. It can be done, it worked for us almost ten years, ending only with the death of one of the partners.I still consider myself bound by it’s terms, even though I’m not legally married to my surviving partner and our relationship is more like a close friendship than a marriage (our sexual orientations don’t match up).
It would be a great idea if you could all simultaneously “propose” to each other (would require a little behind-the-scenes coordination). That creates a nice excuse for a party. Then you could work out the details of the relationship, rules, etc. Write these down and set a date to formalize the agreement and come up with a ceremony consistent with your mutual beliefs (we sealed our agreement in blood). Then you can have an even bigger wing-ding.
Best of luck, I’m really happy for you. I hope that eventually marriage will be defined as whatever it’s adult, legally competent members define it as; with the state serving only as a repository for the agreements and having no role in defining what a marriage is.
PM me if you need details on any particular issues our agreements.