A little clarification before we get going: In July 2016, the Just Cause 3 multiplayer mod JC3-MP was canceled after its lead developer, Cameron Foote, was hired by Avalanche Studios to work on the game itself. A few months prior to that, however, the Nanos team, whose previous work includes multiplayer mods for Mafia 3 and Grand Theft Auto 5, announced that it had begun work on its own Just Cause 3 multiplayer mod, called JC3:MP. And while the first mod remains dead and gone, the Nanos effort is now in full release.
Download @justcause 3 Multiplayer NOW: https://t.co/33Ln0zatZ2More information: https://t.co/dRd4wU6FyAJanuary 6, 2017
The JC3:MP mod gives players the ability to race, skydive, wander around, or blow each other to pieces, all with "state-of-the-art synchronization reducing lag to a minimum." It support hundreds of players per server, playing competitively or cooperatively, and also enables server owners to create their own unique game modes by way of a "powerful scripting API."
The FAQ states that future releases of the mod might be available from Steam, but for now you can download it directly from just-cause.mp. Problems, should you encounter any, can be addressed in the Nanos forums.