1) ISO sesitivity - no meaningful differences except the total range (5D3 beats D800 and D4 beats 5D3)
Just to call this one out, DxOMark's ISO Sensitivity measurement isn't measuring what most people think it's measuring. It has no direct bearing on ISO noise performance (but an indirect one, see below). The name is perhaps misleading, maybe better to call it 'ISO Accuracy' or 'ISO Fidelity'. What this test measures is the real
ISO value (benchmarked against the actual International Organization for Standardization's criteria) vs. the ISO setting on the camera, or to put it another way, it measures how much the camera lies to you when you pick a given ISO setting.
In the plot you can see that for ISO 50 and ISO 100, the dots for the 5DIII, D800, and D4 are all stacked on top of each other, and they're all at ISO 75 for both settings. What that means is all three cameras are lying to you in exactly the same way - whether you set ISO 50 or ISO 100 for your shot, the exposure is actually at around ISO 75 and then pushed or pulled by the camera as needed, although the ISO value you selected is what's actually recorded in the metadata. This lying is not new or unique - both Canon and Nikon do it routinely for fast lenses, where the incident angle of the light exceeds the refracting capability of the microlenses and exposure is 'secretly' boosted to compensate (i.e. about 1/2 stop of the light coming in at f/1.2-1.4 is not detected by a digital sensor, so the camera boosts the ISO half a stop - meaning half a stop more noise - without telling you).
Perhaps of a bit more significance is the way this plays out - if you compare just the D800 to the 5DIII and look at the ISO Sensitivity plot, you can see that the D800 is a little further off the nominal value at all the settings, with more separation at the higher ISOs. In other words, the D800 lies to you a little more than the 5DIII (but the 5DIII is still lying). For example, when you set both cameras to ISO 6400, the D800 is actually shooting at ISO 4211 (it's lying by 2/3-stop), whereas the 5DIII is actually shooting at ISO 5179 (it's lying by only 1/3-stop). Translation - artificial advantage for the D800 because it's shooting at a lower actual ISO than the 5DIII for a given setting.
Bottom line is three fantastic cameras, just take your pick and be happy!
This is the take home message!