I'm not sure I understand.
You need to navigate to your 'my documents, my games, far cry 3 folder' (not the actual game folder)
In that folder there will be a file called 'GamerProfile', open that file with WordPad.
In the file there will be some text, find something that says: ShowFPS="0"
Change the '0' to a '1' and your fps should appear in game.