I have a question: If I reinstall witcher 3 or buy a new computer and then install, can I continue playing when i stopped? From the position I last saved? Thanks for answer.

If you are using the same or a later version of all the components installed, you can use the savegame from MyDocuments\The Witcher 3\gamesaves

Obviously don't delete or overwrite the gamesave if you want to reuse it (but you'd be surprised, this has actually come up as a 'problem' before...)