Well, the low-light ISO scores are 3296, 2293, and 1815, for the 1D X, 5D3, and 5D2, respectively.
The difference between the 1D X and 5D3 is 1003, and the difference between the 5D3 and 5D2 is 478. So, the difference between the former is ~2x as much as the latter.
Or am I missing something...