There's many types of ethical nonmogamy, and polyamory is one possible limb.
(Other limbs on the ethical nonmonogamy tree could include things like casual, recreational sex -- soft or hard swinging. )
But even in that polyamorous limb, there's STILL many branches shooting off it for HOW to BE polyamorous. You seem to be after a cohabitating "V" or "triad" perhaps even going out to calling it a group marriage?
Here's one article about various models.