Look, I hate to tell you this, but you already lost. The first girl in a harem to fall in love and to confess loses, and you did it before the series started. I appreciate your dedication, but you cannot fight the immutable laws of manga.
At least he appears to be not completely dense MC, just a one that reluctant to changes.
I can see so many things that can go wrong with her plan and changes, in fact it already shown up, at the end of the chapter. Is there some kind wordplay on word mood in page 5? It looks to me like he compliment her clothes or style. So it should be
"These three girls all declared their love for and intent to marry me, out loud, then one of them asked me on a date soon after... No, I must be overthinking this..."
Girls explicitly say they like him and to prepare themselves
Guy 'oh they must be joking'
No wonder Japan has a population problem lol, have some more self confidence my guy