If you look on the steam store and hover over "games" from the main page, at the bottom of the drop down menu there is a section called "browse by platform". If you click on "steamOS + Linux", it will only show you games that will work on Linux. You can use this list to figure out whether or not Skyrim and Witcher 3 work on Linux, as well as any other steam game.
For the short version, from this list, we find that neither the Witcher 3 nor Skyrim have a Linux version available on Steam. However, there could also be versions for other platforms that are for some reason not offered on Steam, but I don't know if there are or not.