FE Three Houses has a extremely interesting setting, seems that it will be high quality(at least thats my impression), and it will be able to take advantage of the Switch Effect.
So if we are talking about 2 months worth of sale(since we will get the info in October with sales ending late September), it will easily top 2 million units.Hell, I would go as far as to say that it will pass 3 millions, assuming the game has at least a score of 85 metacritic.
My (locked) thread about how difficulty should be a decision for the developers, not the gamers.
https://gamrconnect.vgchartz.com/thread.php?id=241866&page=1