There's still a chance the Kingdom Hearts collections could make it to XBO/Switch to be honest, and I'd like to think it'll happen at some point... Final Fantasy X/X-2 HD Remaster says "Only on PlayStation" on the cover, yet it's releasing next year on XBO/Switch and is already out on PC. Kingdom Hearts is pretty famous for its ridiculous story which spans over multiple games and is a pain in the arse to follow, so I feel XBO owners won't be getting the full experience and will be left clueless for the most part if they're only playing Kingdom Hearts 3, so for that reason alone, whether it's a good or poor reason, I do think they'll come over to other consoles.
As for sales, I think if Kingdom Hearts 3 were to release on Switch it'd sell better than on XBO just in Japan alone, I mean, it's Nintendo AND handheld, that seems to be a big factor in the Japanese market, although considering the franchise is more associated with PlayStation, I still think PS4 would outsell it there regardless. If I'm to guess an overall split in sales, I'd actually put it somewhere around the 65/20/15 mark for PS4/Switch/XBO respectively. The reason I've picked this split is because, so far according to our physical sales numbers, Final Fantasy XV has sold 5.01m on PS4 and 1.11m on XBO. Of course, it's a completely different game, but Final Fantasy and Kingdom Hearts (and Dragon Quest) are the biggest franchises by Square Enix, so I'd imagine a similar sales split too, but I had to adjust it to account for the Switch too.
I also think Kingdom Hearts 3 will be the best selling game in the franchise, maybe even breaking 10m sales between the PS4 and XBO, simply because it's incredibly hyped up right now, so many people are looking forward to FINALLY playing it after what feels like forever, and this gen we've seen games shoot up into the top spot of their respective franchises in terms of sales, and I can't see this being much different either.