You're absolutely right. I wouldn't mind seeing it explored in fiction and fantasy in particular, which seems to me like the perfect genre to explore those dynamics. The only other story I can think that does it also features a sort of throuple, though the two women have a more sisterly relationship while both are in a romantic relationship only with the male (the Honorverse series)
I hope with queerness and non-heteronormativity becoming more "mainstream" (well, talked about more openly outside of niche spaces) that we'll start to see more of an exploration of what being poly means