https://www.pcgamesn.com/the-witcher-3/sales
44.5% of The Witcher 3 sales came through PC, trailed by PlayStation 4 at 39.5% and Xbox One at 15.5% – all accounting for some rounding funkiness, of course. The info comes from CD Projekt's 2018 financial report, so the numbers account for everything from the game's launch in 2015 until the end of the most recent fiscal year. (Apparently, the folks at PC Games Insider like to dig through old financial reports.)
While PC has grown to be the most popular platform, it was actually in a pretty distant second place back in 2015. If you combine Xbox and PlayStation platforms, that means a slim majority of lifetime players are on console – about 55%.
In 2015, sales were predominantly on PlayStation 4 – 48% over PC's 31% – and it wasn't until 2016 that the PC version managed to overtake the console platforms.