UPDATE: Confirmed by Avalanche on Twitter.
An enterprising fellow ongot his hands on the box art for the PC version of Just Cause 3, which revealed the system requirements.
If it's real, according to the box (which can be seen below) the system requirements are thus:
Minimum system requirements
Recommended system requirements
Just Cause 3 is expected to launch on 1 December 2015 for PC and next-gen consoles.