If you have a saved game file from The Witcher 2 on your PC, "it'll definitely impact your playthrough in The Witcher 3," according to CD Projekt RED Executive Producer, John Mamais.
In a similar fashion to recent Bioware games, you'll be able to import the saved game when starting The Witcher 3: Wild Hunt and this means that certain story details will be changed. The extent of these changes is yet to be revealed.
As for the Xbox One and PlayStation 4 versions, Mamais explained that CD Projekt may need to take a different approach:
"We’re not sure how it’s going to work on the consoles yet, but we might try to create some kind of game state even if you haven’t played. Obviously we didn’t make the game for the Playstation 3, so we might try to deliver something for the PS4 that could create some kind of profile for those players."
The Witcher 3: Wild Hunt is expected to hit shelves next year and will conclude the story of Geralt, the morally-vague monster hunter.