I was almost in one with my bi bud, a female who moved in with us for a while (he was into her sexually, not me) but after about I think it was a year she and he had a falling out and me and she wound up friends but he didn't want anything to do with her anymore. I liked her, she was goofy but real nice. We originally found her and some other friends, picking them up off a seawall while cruising down the Intracoastal. We were a little nutz back then.
We played with lots of others all the time but that was just play. Only one other female tried getting serious with my bud but as soon as she revealed that it was her or me, well, she lost, buh bye. Never saw her again. No great loss. Our dislike was mutual. I didn't even say anything to influence my bud. I knew he'd do right by me and he did.
We never considered another guy for a third. I might have been okay with that but did not require it. I did play with other guys on the side, we had an open relationship. As a polyamorous grouping it would have been mmf with someone who I enjoyed who was also someone who he both enjoyed and was sexually into. It just never happened by chemistry. But we gave it our best shot.