You're basing this on what exactly? If it's the scene with Alex, that doesn't prove anything to be honest. She may or may not be open to sharing, or even a poly relationship, but there has been no actual proof in either direction thus far. Ashe on the other hand, going by the dialogue during her date, it's much safer to say that she wouldn't be open to that sort of relationship. However, she has hinted that she would potentially be down for threesomes.
Mason I think will be the most likely to be open to a polyamorous relationship, at least from everything we know so far. Not sure if he's actually polyamorous, or would only do it for the MC's sake, either way I do feel confident in saying he will be one of the characters that will have that option open. Neon did mention that one of the characters is already a polyamorous person. We don't know who it is yet, who knows, it could end up being a character we weren't expecting. Ashe is the only one I'm confident
isn't that character, based on what we know so far. (Carolina seems to be also unlikely, I won't fully rule it out though.)
Wow Joshy, trying to shatter my dream of a Nicki and Mason throuple are you?