we could have had a really great poly ending, but sadly the manga prob got canceled and had to wrap things out even Sayo and Rinna kiss when Mei got a fever I forgot which chapter was it then a day after Rinna started having a love arrow towards Sayo.
They even have chapters dedicated to poly relationships..
I still wish the author just jump towards the poly ending even if the build up was a bit rush, I'll be okay with it.