maybe i'm going to sound like a broken record, but i'm gonna keep banging on about this.
all three cameras feature high quality sensors operating near the limits as we understand them today. quantum efficiency, read noise, thermal noise, etc.. all likely to be damn close between the three cameras.
the difference will be entirely down to pixel size.. 16MP D4, 18MP 1DX and 22MP 5DIII. difference between 16 and 18 not much at all in pixel area, but difference between 16 and 22 is becoming significant (but not like difference between 10MP and 20MP).
on jpgs, the manufacturers may choose differing levels of in-camera smoothing/noise reduction. but on a RAW file i'll bet that performance exactly mirrors pixel size. shot noise will totally dominate and it's only a function of pixel size.. has nothing to do with anything else in camera.
full frame cameras with large pixels are as good as it gets for low light. and sensors of seven years ago not in the same league, but now they're all very close. collectively we're starting to get a little too sucked in by who's got the lowest noise sensor and the highest ISO.. when you can tell all you need to by just looking at the MP count.
rant over..
cheers
Phil