An interesting article regarding yuri manga that have polyam relationships. Goes into the distinction between harem yuri and yuri with polyam relationships, followed by some examples of polyam yuri. I have read one of those examples (Goukaku no Tame no! Yasashii Sankaku Kankei Nyuumon/An Easy Introduction to Love Triangles (To Pass the Exam!)), and it's an enjoyable read!
I would have just shared AniFem's post for this article since they have a CoHost, but sadly they haven't updated it for a month (and thus they haven't posted a link to the article on here), so I'm sharing it here.