Earlier today I advised a triad who was worried about legal marriage and health insurance. I also want to point out that nothing turns apathy into advocacy better than knowing someone affected by an issue. If you do decide to have a marriage ceremony between any two members of your triad, or if you decide to have a non-legally-binding ceremony, you might want to include a little section in your program that explains your situation. It could be cutesy, or educational, or quick and to the point - but explain why you chose to go the route that you did, and perhaps ask your wedding guests to consider donating to the various organizations that helped you out, or any political candidates whose views on healthcare coverage show promise for plural families.
For example, you could have a page called “Why No One’s Getting Married at This Wedding,” explain the situation in a little story, and then finish with “To ensure that in the future, people like us get to say ‘I Do,’ please consider supporting the following organizations and candidates…” Or, if you and your fiance go ahead and get married after finding another way to provide for your girlfriend’s health care, maybe have her say her own vows as part of the ceremony, explaining how much she loves you two, how true love matters more than legal contracts, and the sacrifices you three have had to make. That would do the double-duty of helping your guests understand your unique situation and helping pave the way for future poly families.
Congratulations on your happy, healthy relationship, and good luck navigating all of this.