Only very few applications are able to really make good use of the performance capabilities of a quad core processor which means that, generally speaking, the performance benefits are definitely not doubled up. The vast majority of programs only support two processors cores and, when this is the case, the computer is simply overworked by the application.
If additional performance is required by the operating system, for the numerous applications running in the background such as Messenger, audio chat, video output on a second monitor or SHH encrypted downloads, the dual core application (for example, a 3D game) loses performance and runs at a slower rate. The third core is ideal for handling these additional tasks. It reduces the load on the other two cores and allows the dual core application to continue operating without disruption.
The Phenom X3 with the three processor cores makes the system capable of multi-tasking, but is considerably cheaper than a quad core CPU from AMD or Intel. AMD, in conjunction with the Phenom X3, is now in a position to offer a considerably cheaper PC system with more functions than Intel.