360 has an ATI GPU, that if I'm not mistaken, is a modified version of x1800 with features similar to DX10.
PS3 has a Nvidia GPU based on the 7800, again if I'm not mistaken.
The 7800/7900 series has been known to produce better raw frames than the ATI one (x1800, x1900), but when we cranked up image quality, resolution, AA, AF, ATI always maintained higher frame rates (and more constant frames). Maybe that this could explain why the 360 performed better in CoD 3.
And the 360 was released in 2005, so it's probably sure that the PC hardware already surpassed the one of the 360. The Oblivion screenshots in this thread illustrate it quite well. So if my hypothesis is true and the Xbox 360 GPU is stronger than the PS3 GPU, it is quite clear that PC hardware is better and more powerful than PS3 hardware.
(Of course, this is mainly speculation!)