People play farcry 3 on ultra 4 core processers like the i5 3570K and phenom 965. farcry 3 doesn't need 8 cores
about the thing about the usage of all cores I don't know, sorry bout that /:
Background programs, and prob each core takes the processing from another core for better utilization from the game as in whatever lets say core 3 is doing gets put to be done on core 5 or something along them lines is what im guessing cause its like not like 4 core are fully are going to be used and the other 4 left near 0% lol
Actually, Far Cry 3 is known to scale well across eight threads IIRC. BF3 MP and Crysis 3 are also good about scaling. It's not rare with some of the newest DX11 games.
It does scale with more cores but you wouldn't see any major differences between a quad-core and an eight-core unless you're using enthusiast builds in which case you probably have a six-core at least. The fx-8350 is probably just as good for FC3 as the 3570k if not worse due to the lack of PCIe 3.0(which is also one of those differences that won't make a difference yet).