Though the Phenom X2 hasn't taken off yet, there is already rumour that AMD plan to add another type of Phenom to their next-gen lineup, the "Phenom X3". Just as the name implies, if this rumour is true then this will be the first CPU to use a 'tri-core' design.
AMD's supposed reasoning for releasing a family of Phenom X3s is that when mass-manufacturing Phenom X4 chips there are regular manufacturing defects with one of the four cores on some processors, rather than scrapping these AMD are able to push them into another sellable line-up.
The Inq have more on this story here.
THERE HAVE BEEN back channel stories floating around for weeks about tri-core AMD CPUs, but nothing solid. The rumours have been picking up more steam, and now they are finally solid: AMD is doing a three-core chip.
The first thing that comes to mind is bad yields, but I am told this is not the case for the three-core product lines. There may be some salvage done on bad quads, but that is not the overriding reason to do this.
The main reason is marketing, it seems AMD is learning from ATI. Most non-top SKU GPUs are simply top SKU die with features turned off, and if you look at the success of people unlocking that, you will see that it is far more than salvage.