In the game launcher please set Display mode to windowed.
IF that doesn't help, Install ALL these Microsoft modules:
www.microsoft.com
www.microsoft.com
www.microsoft.com
www.microsoft.com
(run dxsetup.exe after extracting from this download)
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 those don't help:
DXDIAG is a program you run from a command prompt or the Windows start menu 'run' dialog box (or 'search programs' in Windows 7 or later). After running it will open a window and start collecting info with a progress bar in the lower-left corner. When it completes click the 'save all information' button and save it to a file then attach that file here.
Please attach your Documents/Paradox/CK3/pdx_settings.txt .
From your Documents/Paradox/CK3/logs/ folder, attach system.log , error.log