I must say that the diagrams presented by wr2 are compelling, but if you look closer you'll notice some similarities and obvious confusing details. For example, the smaller i5 clarkdale is very much like a section of the i7. The coloration is obviously artificial and added to identify different sections. The i3 clarkdale is totally different and doesn't make sense at all when compared to the others. It's as though they took an actual photo of the active chip with infrared film, then colorized it like the others. Someone with photoshop could have easily created all of them by moving blocks around to create the i7 and i5 diagrams, but actual photos in block form in the i3. I can't believe that the difference between the i3 core and the others is actually that large using the same die structure. Ironically, the blurred effect of the i3 structure confuses the comparison for some reason because this was intentional.
I initially noticed when I read about the i7 and i5 that the i5 had the same features as the i7, but some of them were 'turned off', which led me to research further. If you do this same research for the i3 you find the same thing, except I noticed that some of the features of the i3 which were not mentioned as being in the i5 or i7 were in fact a part of those other chips when I dug deeper into other descriptions of them all. So other than the apparent size difference and the attempt to confuse you with different parsing of the block positions, and apparent bluriness, these could all be the same chip set with features turn off or disabled or non-functional, however you wish to look at it. There is no proof here or in anything else I've read that they aren't the same, except in the minds of those who wish to believe what Intel tells them.