Back in the early 2000s, key generators (keygens) were real. They exploited weak offline algorithms in old games like Age of Empires or GTA: San Andreas . But game developers learned fast.
Instead of risking your PC's security, you can often find Far Cry 3 for a very low price or even for free through official channels: far cry 3 license key generator
and it is asking for a key you don't have (a common bug), try these official steps: Back in the early 2000s, key generators (keygens) were real