That got cancelled when its lead developer landed a job at Avalanche, the makers of Just Cause. This JC3 multiplayer mod, by the way, is different to the one that was being made by folks behind JC2 MP's mod. The mod also supports scripting for server owners to create and run custom game modes. You'll need to log in through your Steam account and, obviously, own a copy of Just Cause 3. The mod's still in beta, fixes a-flying, but you can now download Nanos JC3MP from its site for free.
This was supposed to launch in December but, following, officially launched on Friday. Just Cause 3 is chaotic with only one play so with hundreds, oh my! The unofficial mods adding multiplayer with potentially hundreds of players per server, chasing and racing and exploding together. That's now resolved with the beta release of Nanos Just Cause 3 Multiplayer.
The big silly sandbox of Just Cause 3 is a fine place to dick about, driving and gliding and exploding all over for giggles, but it was sorely lacking the option to muck about with your mates.
