I'm in what I suppose might be described as a triad or vee (still not sure of poly terms) and it just kind of happened a few years back. I'll use A, B, and C as stand-ins for the people in this relationship and without specifying which is myself. All have differing gender identities, with at least two explicitly considering themselves non-binary.

A, B, and C all currently live together in the same house. A and B are married, A and C are partnered. A and B have separate bedrooms and A and C share a bedroom. A and B were in a previously monogamous marriage for a few years before C came into the picture, and B didn't meet C until months after A and C were dating (A and B had already agreed to allow each to date outside their marriage). B and C get along and are friendly with one another. A, B, and C have family outings together from time to time.

A and B could not have managed this without lots of communication, including the setting of realistic boundaries. B and C are still learning how to communicate directly with each other rather than using A as a relay, and they are getting better at it over time. A, B, and C have sat down many times over the years to have family discussions.

Although this has worked well for us so far, and seems to be getting better over time, it definitely has its challenges. There's a huge element of luck (age, circumstances, etc.) that it's worked out as it has.