More people need to tell them that thrupple is okay. Every chapter, just said by someone random, until they take it to heart.
Maybe a chapter where they can group watch favorite Kiramon form a thrupple?
This is the only manga (except MT if we include LNs) where I welcome a harem ending, and the author pretty much half-confirmed it vicariously through Sayu-chan.
Irrelevant to how we all need them choose to live.
So they just live and bathe in a three person pile of gropey soapy happiness without documentation or family register stuff. This is the way.