Unfortunately that doesn't work that way. Instead of actually sampling the 2 missing channels in a Bayer CFA, they are interpolated. No sensitivity is lost ...
Of course some sensitivity is lost by discarding about 60% of the light arriving at a particular location by absorbing it with a color filter. This leads to lower photon counts than, for example, a monochrome sensor and so worse ratio pf signal to photon shot noise than would be possible if an imagined perfect X3 sensor could count almost all of the incoming photons, binned into counts for three spectrall ranges, "red", "green", and "blue". But the Foveon approach to X3, relying on the fact that silicon has different degree of transparency to different wavelengths of light, loses even more information to absorption of light by the silicon, and so ends up with worse SNR than a good CFA sensor. Particularly for the color measured at the deepest level of the Foveon sensor: I forget whether that is blue or red.
Edit: it is red at the bottom. In more detail, the Foveon approach (not used by the other X3 approaches for which I have seen patents) really takes three measurements with a mix of colors, at three levels in the silicon:
- top: mostly blue, some green, a little red
- middle: mostly green, some red, a little residual blue
- bottom: mostly red, some residual green, a very little residual blue.
Then these mixed signals have to be mathematically unravelled, which increases noise and potentially causes color errors, metamerism and such. Also, the red signal is quite weak: only a rather small proportion of light of any color gets that far through the silicon.