You were able to claim Far Cry 3 completely for free on Ubisoft Connect, but sadly it only lasted 4 days.
In fact, the Ubisoft servers were not working anymore, probably because so many people tried to get their free Far Cry 3 game.
As you can see in this Downdetector screenshot, a lot of people reported problems with Ubisoft.
If you didn’t had the time to claim it, but still want the game, you can get it on Steam or the Ubisoft Store.