Just Cause Multiplayer forums announce "the indefinite halt of JC3-MP development," which sounds like a death knell for this multiplayer modification for Just Cause 3 (thanks DSOGaming). The good news is that this is because the mod's co-lead developer has been hired by Avalanche Studios, so hopefully some of his work will find its way into the main game. The other optimistic note is that this was one of two planned multiplayer mods for the action sequel, and nanos GbR continues to work on theirs. Here's word on what's going on:

Good and bad news often come in pairs, and this case is no exception; several months ago, I was approached with an offer I couldn't refuse: to join Avalanche Studios Sweden as a developer so that I could work on projects present and future. As part of Avalanche, I'll be able to bring many of the things you know and love from JC2-MP into future Avalanche experiences, while reaching heights that a mere mod couldn't facilitate. Unfortunately, due to time constraints, neither I or the rest of the team can actively pursue development on JC3-MP; our lives are just too busy to allow for it right now.

We will still be maintaining JC2-MP where appropriate - while we're not actively adding new features, we will fix bugs, maintain stability, and ensure that JC2-MP remains a quality experience for players present and future. We're still searching for solutions to our most prevalent issues (including AMD compatibility without workarounds); hopefully, we'll be able to make headway in the future.