James, welcome to the forum.
What you are looking for - a long-term committed relationship between 3 people sounds very much like a style of polyamory to me that is often known as a "triad". If you use those terms in a search on Google or your own favourite search engine, I'm sure that you will find all sorts of places where this stuff is discussed.
Please check out The Birdcage - an open, friendly Polyamory forum for all parts of New York State
"Listen, or your tongue will make you deaf." - Native American Proverb