Nobody in this VN is emotionally mature enough to be in a poly relationship for any thing past a short term fling. It would take some drastic growth on multiple characters and you'd conceivably end up with a different story.
Alexis-Dates the MC behind Erynn's back, (even though Erynn doesn't really have a claim on him) is extremely flighty, and I just can't see her being okay with sharing (Erynn included). At best, you get a threesome but that really doesn't mean anything on a poly front.
Erynn-Can't even come to terms with her own feelings about MC, let alone be okay with someone else harboring similar feelings (The Erynn-Alexis rift)
Layla-She gets too jealous. Follow any of her routes where you also interact with others (Val/Tori/Alexis) and watch her looks. She's antsy about it and Elena has to calm her down.
Elena-About the only person I'd accept being mature enough to deal with the headache poly relationships can be. Still, you need more than one character and Layla is the only one she really involves herself with in the story.
Iris-LOL no. She might even love the MC more than Erynn but she puts up that façade of being the cool ex.
No one else is even worth discussing from a poly front.