User _darth_solo on Reddit claims that Bayonetta 3 is releasing in 2018. His claims are somewhat noteworthy since he contacted the Mods of the Nintendo Switch subreddit to provide evidence of his claims and recieved their approval. 

_darth_solo goes on to claim that the game could release as early as summer 2018 and it will likely be showcased in the upcoming Nintendo Direct (although the mods pointed out that he had no evidence of the game being part of the showcase) and won't come out later than 2018 barring a disaster in the last months of development. The user then goes on to say that this Nintendo Direct will be "Nintendo's E3" (again, no proof of this, just that the game is coming out in 2018).

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I don't want to get anyone's hopes up, especially mine, but this guy seems legit...(if not the the full NintendoSwitch subreddit shall burn)

Even if it's not, I can replay Bayonetta 1 and 2 again on the go.

Platinum were said to be working on a at the time unannounced project early last year which we can now assume was B3 so I wouldn't rule it out.

im pretty sure bayo 3 will be in 2018...

2018, yes. Summer 2018, definitely not.

I think Bayonetta 3 will most likely be their big September game with a simultaneous worldwide release. But, I can't envision anything sooner.

Good to bolster that 2018 library.

It would make sense. Bayo 2 released a while ago, maybe around 2014-2015 Nintendo contacted Platinum to work on Bayo 3 for the NX.

The problem is no gameplay has been shown at all. Until I've seen gameplay I'm not convinced. We have no idea how long they've been working on Bayonetta 3 because they haven't announced a release date and had just shown a really short cinematic trailer and nothing else.