Right click on ck3.exe , properties, compatibility. Tick "disable fullScreen optimisations" untick every other box in that dialog, Apply and exit.
Then in the launcher Game Settings menu, select fullScreen display mode, Vsync off, refresh rate 60.
IF that doesn't help, Please go here:
This article lists the download links for the latest versions of Visual C++ Redistributable packages.
support.microsoft.com
And run the x86 and x64 installers there.
IF that doesn't help, Has the game ever run OK on this computer? Or never?
IF NEVER:
Install ALL these Microsoft modules:
www.microsoft.com
www.microsoft.com
www.microsoft.com
(run dxsetup.exe after extracting this download to its own folder anywhere on your computer)
Add CK3.exe to the exceptions list of your antivirus app; ESPECIALLY if you have Windows Defender, add it to the Ransomware "Apps Allowed Through" list.
IF IT USED TO WORK:
Please do a full **clean** re-install:
- move any valued save games elsewhere
- "uninstall" in Steam-CK3
- manually delete both the Steam/SteamApps/common/CK3 AND Documents/Paradox Interactive/CK3 folders
- re-install game in Steam, run a Steam Verify when done.
- add CK3.exe to the exceptions list of your antivirus app; ESPECIALLY if you have Windows Defender, add it to the Ransomware "Apps Allowed Through" list.
- start the game with no mods active and test
In particular make sure there are no files or folders left under either the Steam CK3 or Documents CK3 locations, before installing again.
If those don't help:
Please attach here your Documents/Paradox Interactive/CK3/pdx_settings.txt .
From your Documents/Paradox Interactive/CK3/logs/ folder, attach here system.log , error.log , exceptions.log