There are several reasons why you would want to be able to find your save files if you are a Just Cause 3 payer. Among the popular reasons, one is to generate backups that prevent progress loss in case saves are lost or corrupted. These backups are doubled as saves; you can get back to it at a later date for mere nostalgia or to check how much of progress you have made in the game. Certain third-party applications that let you change your game saves also need access to the save files. That is why it is important to know where your saved files are.
But the question should not be why you would want to access your game saves. Rather, it should be how you can access them. For your operating system, you can get help below:
%UserProfile%\Documents\Square Enix\Just Cause 3\Saves
To open that path up, just open File Explorer (right click your start menu button in the bottom left hand corner of your screen and click File Explorer), paste it into the address bar at the top, and hit ENTER!