If your build requires a network fix, copy the fix files (often including modified .dll or executable files) directly into the main game folder where the original Sam3.exe resides.
Once the base game is installed, follow these steps to enable online play: download-serious-sam-3-bfe-build-09052016-online
Find your installation directory. By default, Steam installs it under Steam\steamapps\common\serious sam 3 . If your build requires a network fix, copy