Just Cause 2 Multiplayer Mod was an incredible piece of work that took the modding team years to get to a functioning state but once it was there it turned out to be really, really fun in that special Just Cause 2 sort of a way. 

Luckily for us it would appear that the engine Just Cause 3 runs on is very similar to the old one because the modding team behind JC2MP has already come out with a teaser trailer showcasing seemingly flawless Just Cause 3 multiplayer.

There are some limitations to this however. Much like how it worked in JC2MP there will be no NPCs roaming the world due to the amount of complexity they add nor will you be able to deform the terrain by doing things like blowing up bridges and so on. Other than that you should be capable of doing most of the things you can in single player.

If you're an avid JC2MP player you need not worry about the mod being abandoned because the creators are planning to support both mods for years to come. How these guys haven't been hired by Avalanche yet is completely beyond me...

As of right now JC3MP is not ready for public release and as such there is no download link for me to share with you. But given how well done the interactions between the players in the trailer seemed I have a feeling it will be arriving pretty soon and when it does I'll make sure to let you know.