Hello! I have bought the Witcher 1 and 2 games on the Steam platform, and I have played them both and finished them. Now I want to play Witcher 3 version, but I want to buy the game on the GOG platform. My question is that if I will be able when I start the Witcher 3 game, to import the saves from Steam (for witcher 1 and 2), so my choices and other stuff regarding the story will also be imported in the GOG version of the Witcher 3. Thanks!

Yup. Saves are in the same place regardless of platform. If the game can't find your save, maybe move them. In the Witcher 3 forum there's more info on that.

Don't forget that, if you own the first two games on Steam or otherwise, you are eligible for a free backup copy from this site. Just go to
https://www.gog.com/witcher/backup
and (after making sure the connection is encrypted) enter your product key.

Thanks a lot guys for your answers :D I just bought Witcher 3 today (on GOG), and I can't wait to finish the first two games and then play the third one. Have a nice day! :D