Not really, no. Traditionally multi-CPU systems have always been in even numbers, but there's really nothing that says you can't have an odd number.
Im in the market for a new PC, and the one with the video card I would like happens to be a triple core processor, AMD 8550.
I was only aware of duals and quads, I had no idea triples existed. Is there any reason why I should not get a triple core? Im looking for some insight.