The Xbox One version of the game was the lowest selling.
The Witcher series, now ten years old, has sold 33 million units worldwide, CD Projekt RED confirmed in its financial report for 2017. Given that the franchise started as a niche, PC exclusive low budget RPG, this end result is staggering.
Most of the sales obviously come from the seminal The Witcher 3: Wild Hunt, which launched on PS4, Xbox One, and PC in 2015. We got a breakdown of sales for that game as well, with CD Projekt RED confirming that the PC version of the game is the highest selling one, followed by the PS4 version, and finally, the Xbox One version having sold the least (which is interesting, because Microsoft had the marketing rights for the game). Interestingly enough, the PS4 version actually outsold the PC version of the game in 2015, though the PC version more than made up for that in the next two years.
Given just how fantastic a game it is, The Witcher 3 has had a long tail, and will probably continue selling for a long time to come.