Wrong again, KenV.
Regarding the Foveon X3 Sensor:
Originally Posted by Wikipedia
It uses an array of photosites, each of which consists of three vertically stacked photodiodes, organized in a two-dimensional grid. Each of the three stacked photodiodes responds to different wavelengths of light; that is, each has a different spectral sensitivity curve. This difference is due to the fact that different wavelengths of light penetrate silicon to different depths. The signals from the three photodiodes are then processed, resulting in data that provides the amounts of three additive primary colors, red, green, and blue.
The photodiodes are stacked, not three individual "sensor pixels". Sigma refer to them as "x3" because the 15MP sensor uses 4600x3200x3 photodiodes, but only 15ish million photosites.