The saves will live in ":\Program Files(x86)\Steam\userdata\[userid]\20920\remote." Simply copy the files here to ":\Users\[username]\My Documents\Witcher 2\gamesaves" and you'll be given the proper option to import a Witcher 2 save when starting a new game in The Witcher 3.

How do I use Witcher 2 Save?

When starting a new game of The Witcher 2: Assassins of Kings, you can import a saved game from The Witcher by selecting "New Game" then "Import Witcher 1 Save" from the main menu.

Where do I find my game save files?

Your saves can be found under the AppData\LocalLow directory. Once there, enter the folder of game you were playing. Inside, the Save game should be named SAVE_GAME.