I'm all for poly if it makes senses in the relationship.
Like any relationship, a poly one takes effort to make work. Relationship that just work magically without any effort or concessions are excessively rare.
Canno's explained the process of how her characters end up in a poly relationship and that was well done.
Here, I though the MC leaned toward the childhood friend, but if it ends in a 3-sided poly, it's a bit out of the blue and maybe indicative of the ax.