If you are on different networks or having trouble with local detection, software like or Radmin VPN creates a virtual local network that games can use even without a standard internet connection.
The most reliable way to play Serious Sam 3 over LAN without Steam is to use a Steam network emulator. These lightweight programs trick the game into thinking it is connected to the Steam network, redirecting all traffic to your local area network instead. Popular choices include Goldberg Emulator or SmartSteamEmu. Step 1: Download a Steam Emulator how to play serious sam 3 multiplayer lan without steam hot