It's not MMF, but FF(F)M, but if you're interested in that kind of subject Hare-kon has surprised me in being pretty good at describing a legitimate poly-amorous relationship. Like I thought it was just gonna be your typical run of the mill harem with plenty of tits and ass (which there is a lot of that but...) I was surprised at how the series fleshed out the characters. It's not just hot dude surrounded by chicks, the multiple wives of the relationship actually have relationships with one another (albeit, not exactly romantic/sexual) and it's not just about pleasing their husband. It goes into the legitimate difficulties of poly relationships; jealousy, boundaries, ensuring everyone gets adequate and quality private time, etc, and it handles them relatively nicely while maintaining it's status as a drama.
Granted, I could be talking out of my ass as this is not my lifestlye nor one I would want for myself anyway, but just reading this from an open minded point of view and comparing it with the smut of the genre, it is kind of refreshing to read such an overall positive and realistic piece of work about the subject. I know it's not exactly what you're looking for, but I do think it's a pretty good series with poly-amoury as a plot point. I'm not sure how the direction of the manga will go, part of me is skeptical that it's not going to end with the MC and her husband being the last survivors of the marriage, but the latest arc has been shaking those doubts from me.
