Question about processor cores

Lets say I have a quad core proc (I have an i5 2500K CPU), and I run a 3-threaded game like starcraft 2. will the cpu automatically use the the leftover core to run processes in the background or is there a designated order of which core can run what (for example, core 0 HAS to be running background apps, core 1 HAS to handle a certain task etc.)?

As far as I can tell by watching my processor usage I'd say yes, it does a good job of spreading things around to all 4 cores. I wouldn't quite say things are fixed to a core though. You will see the benefits but how it divvies itself up is probably complicated. With 4 cores the CPU rarely gets fully taxed in a game.