Save the file and launch the game via Steam. If it boots successfully, adjust the settings to your preferred fullscreen preferences in the in-game options menu. 📦 Fix 2: Install Legacy DirectX 9 Runtime Components
Navigate to the Serious Sam 3 app ID folder, which is . serious sam 3 cannot set display mode
This should be the first line of attack. The game ships with the correct version of the DirectX installer, but it often doesn’t run automatically. Save the file and launch the game via Steam