From the game manual:
Importing Saved Games
You can import a saved game from The Witcher® 2: Assassins of Kings.
Doing so will incorporate the choices you made in that saved game into your The Witcher 3 experience.
To import a saved game, click on Import The Witcher 2 saved game in the Main Menu under the New Game option.
NOTE: This option is only available if there are saves from The Witcher 2: Assassins of Kings present in the saved game folder:
%SystemDrive%\Users\%USERNAME%\Documents\Witcher 2\gamesaves
Any The Witcher 2 saved games present in that folder can be imported into The Witcher 3: Wild Hunt.
However, it is highly recommended you use the final autosave from Act 3 of The Witcher 2, as any earlier saves will not incorporate choices made in the latter stages of the game.
Default settings will be applied to any choices not imported.
If no The Witcher 2 saved games are present, a dialogue can be triggered during the Prologue, allowing you to answer questions to simulate an import of your choices from The Witcher 2.