Do I have to buy the 3 Add On's to play the difficulty levels in Nioh?
(Dragon of the North + Indomitable Honor + End of the Bloodshed)
Do I have to play these 3 missions to get to the next level of difficulty (Way of the Demon)?
To unlock "Way of the Demon", you must:
play through the game once
to change the difficulty to "the way of the strong"
complete all major missions with difficulty "Path of the Strong"
Source: https://www.gameskinny.com/xzuls/nioh-dragon-of-the-north-how-to-unlock-way-of-the-demon