Sorry, I can't really help you, but the manga Six Sex has a three-people relationship. But if I remember correctly, it's not translated, but you can easily find raws. Also, it's about mafia and I can't really tell since it's in Japanese, but it doesn't look that 'heathy' though it's definitely better than some other stories with this kind of relationship.
I probably didn't help you at all, right :D
I saw this on Lezhin today: https://www.lezhin.com/en/comic/polyamory_en
I have no idea though if it's just smut or actually about polyamory
>_> Actually I'm part of a scanlations group so I'm gonna just... Suggest this one... Casually. For a future project haha. Thank you!
It looks like it'll be smut, but I'm still interested! Thank you!
I'm probably asking for the impossible, but please recommend manga (yaoi, yuri, shojo, etc) that features at least some semblance of a healthy poly (3+) relationship?