If anyone else is wondering about the Retail PC version, this guy on Reddit says it comes with a Download code and 5 Disks to install from!!!
https://www.reddit.com/r/JustCause/comments/3uh0cq/just_cause_3_pc_edition_explained/
https://www.reddit.com/r/JustCause/comments/3uh0cq/just_cause_3_pc_edition_explained/
The PC edition came for me with in the box a game-code which will unlock the game on Steam, plus 5 disks. This means that if you buy JC3 physical, you can chose whether you download the game, or install the game from the disk.