According to Steamspy, there are almost 600,000 owners of the Witcher 3 on Steam.
Then you have to factor in GoG and retail sales. I think it's safe to assume that there are at least 500,000 copies sold on GoG, then probably about a 100,000 for retail copies.
All in all, I'd say PC has up to 1.5 million copies sold. PS4 likely has a bit more at 1.8m and Xbox One has the rest.
Or the numbers for PC and PS4 could easily be reversed.
But in the end, PC will create more profits for CDPR due to:
1) Digital dominance. I believe devs earn more per digital copy sold than retail.
2) GoG which means CDPR pockets the entire cost of the game without sharing it with Sony and Microsoft.
3) PC platform has much longer sales cycles than consoles, especially if the modding community does a good job with the game.