A Steam forum post outlines the complete minimum and recommended system requirements for Just Cause 3, just cause (thanks PCGamesN). Here's word what it will take to run the open world action sequel:

Minimum Specifications
OS: Vista SP2, Win 7 SP1, Win 8.1 (64-bit Operating System Required)
CPU: Intel Core i5-2500K, 3.3 Ghz | AMD Phenom II X6 1075T 3 Ghz
Memory: 6GB RAM
Graphics: NVIDIA GeForce GTX 670 (2GB) | AMD Radeon HD 7870 (2GB)

Recommended Specifications
OS: Vista SP2, Win 7 SP1, Win 8.1 (64-bit Operating System Required)
CPU: Intel Core i7-3770, 3.4 GHz | AMD FX-8350, 4.0 GHz
Memory: 8 GB Ram
Graphics: NVIDIA GeForce GTX 780 (3GB) | AMD R9 290 (4GB)