unity 3 support multi-core CPU ?

Unity 2 already does; some parts (like sound? I think) run in separate threads. Your code can also support multi-core CPUs by using threads (System.Threading), although it's somewhat rare for game code to be easily multi-threaded, not to mention that working with threads can be quite tricky. Also no Unity functions are thread-safe, and will crash if used in threads, so you have to stick with standard Mono functions only.