Welcome - I actually don't think that the setup with a "hinge" and others not being in multiple relationships is rare at all - a lot of folks are in a "V" - yours is just some letter that hasn't been invented yet - maybe a "Y"?
How well do the three of you get along on a day-to-day basis?
Welcome to the forum!
"Listen, or your tongue will make you deaf." - Native American Proverb