In which world? Sure, the game and the story is pure fiction, but the devs tried to make the characters realistic. No woman in their right mind accepts a throuple as a serious relationship. That's just porn fantasy.
Studies about it are somewhat limited, but in actuality poly relationships are much more common than you might think. About 1 in 9 people have been in a polyamorous relationship at some point in their life, about 1 in 6 desire to at least try it, and about 4-5% are currently in a poly relationship (based primarily on studies in the US and other similar countries, source
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). That actually makes poly relationship about as common or even more common as compared to gay/lesbian relationships (about 3-4%), so poly relationships statistically aren't any more "porn fantasy" than lesbian relationships. Anecdotally, I personally know 3 women in two different long-term poly relationships (that have been together about 15 years and 9 years).
However, there's still huge social stigma and a lot of legal issues that make being open about poly relationships very difficult, so most people in that type of relationship keep it hidden from the public for fear of being ostracized. That's also a big reason why you don't see a lot of positive representation in popular culture like movies and TV shows; investors and studios don't want to risk public backlash.
What's interesting as far as how it relates to Chasing Sunsets is that for many people entering into their first poly relationship, the way the MC/Jaye/Mal one forms is a fairly common situation. That is, when two people in a committed relationship decide to open it up to a third person (either as a vee or triad). Arguably, MC/Jaye are not in a long-term committed relationship since they never acted on it until now, but you could also say that the feelings have definitely been there a long time. Narratively, I suspect that's what the devs were going for with the Jaye/Mal poly path.