The last chapter of volume 3 (chapter 20), rather than having a chapter number, is marked 最後話, or "last chapter." The chapter does not have a "the end" marking, but the last page is followed by an Afterward by the mangaka. And chapter 20 does end with a significant advance in their relationship (PG-rated), though without translating it I cannot tell whether it resolves the love triangle problem, as the characters we see in that last page are Hasumi and Kisara's ghost self.