I feel like a total goober asking about this; I know this isn't factual research. Usually I can find all kinds of great things on Google about wide ranges of experience I have little idea about. This one is an issue, I'm only coming up with guides to overcoming unreasonable jealousy or bizarre stories of extreme behavior. Which is interesting, but not giving me much to draw on for my (mostly) emotionally well-adjusted characters that take monogamy a mite more seriously than I can manage.
It's a "standard fantasy setting", the kind of fantastically standard smorgasbord of historical elements with no direct equivalent to any earthly time or place while maintaining that nonspecific medieval flavor. Just about any old-timey preindustrial technology can fit in somewhere. Hetero-monogamy is the norm, but taboo relationships will get a condescending eye-roll and not a witch burning. More modern than would be appropriate for a very accurate medieval flavor. Some of the characters come from places where that might vary a bit.
These are the characters, if this context helps, which I don't think it will.
We have a gentleman and a lady who are both attracted to the same lady, and neither relationship has yet been consummated. Our protagonist is the most cynical of these three. She's sharp but distrusting and withdrawn. She is responsible for leading the group and falls easily into a protective role & the two attracted characters like feeling looked after.
Our gentleman is more shy and insecure. This is probably his first crush felt so deeply. When he's feeling confident he hides sensitivity with sarcasm or playfulness. Since he is so shy, she doesn't realize how deeply he feels. He's naive and takes criticism too hard, or sometimes just imagines it. Usually pretty easy-going, except when feeling inadequate. Tends to be a buttmonkey.
This is the guy I'm trying to get a handle on. He brings it up and forces her choice.
The lady is a little older, charming, much more worldly and seductive than the other two characters. Despite her command of sexuality and willingness to use it as an ends to means (whereas he would not) she's actually much more of a romantic and wants to be swept from her feet. Tends to be frivolous and/or superstitious. A magic pixie dream girl, to be perfectly honest, although that may even be a role she consciously adapts due to its appeal.
Both she and the main character are very bookish and are the most well read of their friends.
Also flirty (although not Poly, which the main character is) so she's less threatened by competition at these early stages.
If it matters: she ends up with the guy. He got there first & they had to rely on the other for a long time. When forced to choose, she is much more trusting of him.
Edit, re: naamah_darling
I think it will help the rest of the comm answer your question if you gave us a mini-paragraph about how your playful gentleman and your sexy pixie relate to each other!
Although they're both introspective characters, she is more so. Definitely calmer and less prone to worry. Pixie immediately comes off as flighty, romantic, and likes to be regarded as a woman of mystery. In conversation she's a little fickle, but she's more loyal and sharper than a harsh judge would pin her. I used Joan of Mad Men as a model for the sort of sexuality she carries, influenced heavily by what appeals to others (men especially, even though she's bisexual) while still being savvy and perhaps a little manipulative. Has a talent for intrigue, tends to use sexuality and charm to get ahead in a world where men have an edge. Enjoys pageantry and ritual, the sort of thing that is boring to him.
Although he has stronger moral imperatives which cause him to see her manipulative talent as a bit distasteful, overall he's very trusting and doesn't doubt her good intentions. Sometimes he'll find himself being mocked (good-natured) if they find themselves in a situation where she catches onto something subtle before he does.
They would normally along pretty well, and it's on his suggestion that the protagonist decided to take a risk and trust this woman who was once a stranger. Since it's become apparent to both that their mutual attraction is also attracted to both of them, I wouldn't say things have become hostile between them. He's too bashful and she almost enjoys the competition and watching him be so awkward about it. Some playful competitions have cropped up; apparently she holds her alcohol a little better. Since he's less confident I'd imagine he's more unhappy about the whole thing. He'd be snippy here and there, but never quite in a way that is easily escalated. Overall, each are pretty career focused and the romances would be dropped if things got too tense.
I like talking about my characters so apologies for excess detail.