Known as JC3:MP, the mod allows for hundreds of players to join together on a single server to play competitively or cooperatively. You can explore and destroy the world together, or take part in races, deathmatch, and other activities. Here’s a trailer released last month, when the mod first entered beta:
Work on the mod has not yet been completed; an FAQ states a Steam release will eventually happen. For now, you’ll simply need to download the mod and run the new launcher. System requirements are no different from those of the base game; you’ll only need a copy of the game on Steam to play.
It previously looked as if multiplayer would not happen for Just Cause 3. In 2014, the team behind the incredibly fun Just Cause 2 multiplayer mod announced a follow-up for Just Cause 3. It was then canceled last summer when lead developer Cameron Foote announced he had been hired by series developer Avalanche Studios. As such, development on his mod, named JC3-MP, ceased. The unrelated JC3:MP project now takes its place.