AMD used to have a 3 core desktop CPU. IIRC it was because one of the intended 4 cores was bad but they could salvage the package by making it 3 cores. The 3 core option was so popular, that they kept it once the 4th core issues were resolve with a software patch. Clever people figured out how to enable it once installed and got a 4th core for free. Never used it myself, and only heard tales about it. Could be urban legend.