The Xenoblade 2 argument is rather silly and simplistic...
How many games have Monolith Soft worked on ever since they released XCX? Did you perhaps forget to compare that to the number of games Platinum Games developed or are still developing since Bayonetta 2?
1. The Legend of Korra (That one released just after B2... the result was a bad game)
2. Transformers Devastation (meh)
3. TMNT (bad game)
4. Star Fox Zero (meh)
5. NieR Automata + DLC (Platinum's savior!)
6. Bayonetta 1 PC and Switch ports/ Bayonetta 2 Switch port
7. Granblue Fantasy (looks amazing so far)
Xenoblade 2 coming out early surprised me, but it wasn't only unoptimized and rushed according to fans, but the team simply had all the time in the world to focus their attention and resources onto this one single title. Plus, it apparently took around three years to develop.
I don't see Bayonetta 3 coming any earlier than mid-2019. Late 2019 is more likely.
you're sort of basing what you're saying on very little. you don't KNOW when Bayonetta 3 was started. For all we know Nintendo might have made a deal with Platinum immediately after Bayonetta 2 released and got rave reviews.
Also towards your Xenoblade Chron 2 comments- the game runs very well, it's just portably where it struggles a bit, but that's the reality of trying to put a big game on a portable console, sometimes there will be tradeoffs.
Nintendo has been pushing to do short announcement to releases lately and the reality is that they almost froze releases during the last 2 years of the Wii U. There likely definitely are plenty of titles that are quite far along that could be released in 2018. Again, Nintendo did nothing for like the last 2 years of the Wii U as far as releases.
I am uncertain about Bayonetta 3 coming out in 2018, but there's currently nothing to base an argument on saying it's unlikely. There is a strong argument regarding the fact that they're releasing the compilation of 1+2 pretty early. Generally speaking studios don't release those THAT far out from the main new game because they want to pressure people to buy the bundle before the new game comes out without a big distance of time
in the end until we get some words on when development started on Bayonetta 3 it's tough to say either way. It also helps that, as far as we know, Platinum isn't working on a huge load of titles at the moment