I do not doubt the practice in fiction or reality. My objection is to that made up bullshit word! This is why we still need books in HARD copy. Idiots can make the internet say whatever the fuck they want. Then kids read that made up bullshit, take it as fact and perpetuate it.
Have you ever seen the Mike Judge film "Idiocracy"??? It was produced as a farcical comedy, then it turned prophetic and now our culture has physically and mentally started to emulate it. The film was intended to be a comedy but the basis of the show, the world growing more and more idiotic, is coming true at an alarming rate! When I first saw the film, I laughed my ass off... Then I started seeing the reason Mike Judge had the idea in the first place... it should be in the dystopian sci-fi horror genre now!
I am not criticizing anyone specifically here in forum today. Just giving a word of caution to take care what you emulate and perpetuate back into society. Your example of the MMF relationship reminds me of one of my favorite films, Savages from 2012 with Aaron Taylor-Johnson (yeah, the Kickass guy), Taylor Kitch and Blake Lively as the trio of lovers and an all star supporting cast including Benicio Del Toro, John Travolta, Salma Hayek, Demián Bichir and Emile Hirsch. You won't hear that ridiculous made up word used once.
I have seen a lot of three way romances in fiction through the years yet this new bastardized contraction of threesome and couple is a rather recent development. It's a misguided concept like trying to add a new digit to binary code or a fourth side to a triangle. It's just fucking stupid. Sometimes it works to do that but this one is even worse than spork! Just my two cents, humble opinion from one who has spent a lifetime loving, cherishing and enjoying the English language.
Adventure ever on my friends, Phat