Crisis Core Reunion released on all platforms simultaneously. The Switch version sold terribly, even losing to Xbox in some regions iirc.

Compared to that, this game opened well considering it's a late port on a small install base. But as always, I wish we knew how much of these initial sales were from hardcore fans double-dipping. Cheap launch price, portability, and being a very late port encourage double-dipping.

As for how much it sold... I would guess it was at around 9 million before the Swirch 2 and Xbox release when the Twin Pack is accounted for. This was actually how I got Remake; I orignally intended to buy all three games when the 3rd one is out, but the Twin Pack deal changed my mind. I wonder if there will be a Triple Pack preorder with all three games included for the price of one.

Yep, exactly. I hope SE have it ready for September. I don't expect much simply for the same reasons Rebirth underperformed on Playstation, but the sooner they can get it onto Xbox/Switch2, the more likely fans of part 1 are to follow up on their purchase. 

Then finally the trilogy should be able to wrap up fall 2027

Yes. Square never gave any numbers for Rebirth. I think many people were a little disappointed by part. 1 so they didn't take part. 2

They really should have done a remake in one game.

Yeah I don't remember seeing any actual numbers for Rebirth, though it was reported shortly after launch that it was selling about half as well as Remake:

https://gamingbolt.com/final-fantasy-7-rebirth-has-reportedly-sold-about-half-of-what-ff7-remake-did-in-the-same-timeframe