The Witcher 3: Wild Hunt was a huge success and by far CD Projekt Red's biggest game to date. The biggest contributor to these sales are PC which, according to the company's CEO, Marcin Iwinski, sold better than PlayStation 4 and Xbox One - combined.
We don't get any exactnumbers though, but one of the reasons for might be the fact that the series started out on PC exclusively, the second game came to consoles but only Xbox 360, and it wasn't until The Witcher 3: Wild Hunt that the PlayStation fans got to enjoy Geralt's adventures. So the established fanbase on PC very strong to begin with, comparatively.
Another explanation for the huge difference is that The Witcher 3: Wild Hunt first and foremost a PC game with marvellous graphics that just isn't possible on consoles, more so than any other game. It is also cheaper on PC and is sold through CD Projekt Red's own digital download store GOG.com, something they've heavily promoted.
Thanks, Gaming Bolt.