Simply delete SeriousSam3.ini . The game will generate a fresh copy with safe defaults when you restart it.
Open the file with Notepad. Look for these lines under the [Display] section: serious sam 3 cannot set display mode
RefreshRate = 60 If you have a high-refresh-rate monitor, use its actual supported rate (e.g., 144). Also ensure Monitor is set to 0 (default) or the correct index. On laptops with both integrated Intel graphics and a dedicated NVIDIA/AMD GPU, Serious Sam 3 may default to the weaker Intel GPU, causing mode-setting failures. Simply delete SeriousSam3
%USERPROFILE%\Documents\My Games\Serious Sam 3\ (Paste this path into Windows File Explorer’s address bar.) Look for these lines under the [Display] section:
Width = 3840 Height = 2160 RefreshRate = 60 Fullscreen = 1 Set Fullscreen = 0 to force windowed mode temporarily. Also, change Width and Height to your monitor’s native resolution (e.g., 1920 and 1080). Save the file and relaunch the game. If you can’t access the config file or prefer a quicker test, add command-line parameters to force windowed mode.