Far Cry 3 Blood Dragon has been confirmed as a standalone title coming to PSN/XBLA/PC in the coming weeks. You will not need Far Cry 3 to play it.
Here is the official description along with some screens:
Far Cry® 3 Blood Dragon is THE Kick-Ass Cyber Shooter taking place on a bizarre open-world island crawling with evil. Welcome to an 80’s VHS vision of the future. The year is 2007 and you are Sargent Rex Colt, a Mark IV Cyber Commando who’s fighting against a cyborg army gone rogue. Your mission: get the girl, kill the baddies, and save the world. Experience every cliché of a VHS era vision of a nuclear future, where cyborgs, blood dragons, mutants, and Michael Biehn (Terminator, Aliens, Navy Seals) collide. Playing Far Cry® 3 Blood Dragon doesn’t require a copy of Far Cry® 3.