In File Explorer go to where Steam is installed, typically c:\program files (x86)\Steam. From there go into SteamApps and Common. If the CK3 folder is still there after the uninstall, delete it.
Then click on Documents in the left column. In the right pane go into Paradox Interactive, CK3. Delete everything in it except the save games folder.
Then click on Documents in the left column. In the right pane go into Paradox Interactive, CK3. Delete everything in it except the save games folder.