Steam\userdata\[YourUserNumber]\41070\local\SeriousSam3.ini .
The fastest way to bypass this error is to force the game to start in a window rather than defaulting to a broken fullscreen mode. Open your . Right-click on Serious Sam 3 and select Properties . Under the General tab, look for the Launch Options box. Type in: +sam_bfullscreen=0 Close the window and launch the game. serious sam 3 cannot set display mode