Because the more the merrier.
You can be in a traditional triad (a three person relationship) or something more complicated involving innumerable people and still manage to find a costume that perfectly expresses your group dynamic.
I don't know what your experience has been, but mine has often involved the stereotype that people in polyamorous relationships are humorless about it. The reality couldn't be further from the truth. Poly people love to talk about the highs, lows, and hilarity of being in committed relationships with more than one person.
1. The Polyphonic Spree
The Polyphonic Spree is a band primarily known for their one hit, and their cultish choir robes. The more the merrier for this costume, provided you've got easy access to brightly colored fabrics and a friend who is a whiz with a needle and thread.
2. Robert Palmer and the Addicted To Love girls
If you're in a poly relationship with a dude who refuses to do Halloween, this is a good option. It's hard to get more minimal that Robert Palmer's look in this video, plus you get to look sexy as hell which as we all know, is the goal of all Halloween costumes. (THAT IS A JOKE OH MY GOD CALM DOWN.)
3. Pretty Little Liars
The romantic highs and lows of the Pretty Little Liars make polyamorous relationships seem totally tame by comparison. You can be Aria, your dude can be Ezra, and your lady can be Fitz's girlfriend who we all thought was murdered by a Columbian drug cartel but is actually alive! Alternatively a solid Hannah-Spencer-Caleb triad is a hot, hot choice. This goof assumes you've watched all of PLL and I am 100% fine with that, y'all.
4. Mario Brothers
Part of the beauty of being polyamorous is getting to know such different people. There's always going to be someone who wants to dress up like a princess, and there will always be someone who wants to be the plucky plumber. Also there are mushrooms and gold coins. The video game nerds of the relationship are just as happy as the abstract expressionists. It's really the perfect poly costume.
A poly relationship isn't about separate couples, sure, that's one aspect. But in poly dating the whole is so much greater. What better way to express this than by dressing up as the building blocks for all human life? Separate, okay, but altogether, exceptional!
6. Big Love
This one is a little on the nose, but your friends are sure to appreciate that you can poke fun at yourselves when you dress up as HBO's best drama about plural marriage, Big Love. Plus, Bill Paxton is hot as hell, you know I'm right.
7. Donald and his wives
It is election season, after all, and whether you are for Donald Trump or against him, what better way to celebrate the man and his life than by dressing up as Donald and Melania? And Donald and Marla? And Donald and Ivana? Pobody's nerfect amiright, y'all?
8. Bill and Hillary and Monica
And since I am a bipartisan journalist (lolol) you could always kick it old school and go as Bill, Hillary, and the infamous Monica Lewinksy! Just try to keep it classy and make it beret-Monica not semen-dress-Monica. Taste is essential.