Green098 on 15 June 2017
Since it's on 3 platforms it's best case scenario that is probably possible is for it to just barely skim 3 million, although even that is not a great number for what is a main series Monster Hunter title, especially for a multi-plat title and even then a think it'll end up closer to 2 million.
It'll sell the best on PS4 because of it being exclusive there in Japan, but it'll likely sell the best on PC in the west and Xbox One sales will be the overall lowest.
World Wide
PS4 - 1.3 million
PC - 500k (don't read too much into this since I barely have a clue about sales when it comes to PC)
Xbox One - 350k
