I own both, 1DX and 5D3. Let me say this. At high ISO, say 12,800, I can still print 8 x 10's or better with very fine detail and no noise with the 1DX with minimal processing, whereras the 5D3 requires a lot more post processing. Also, the 1DX files can pull more shadow detail and highlight recovery. Despite having 3 less MP's, the prints are substantially better. My Nikon buddies who shoot sports, ie D4, say they wish they had a 1DX instead when they look at my RAW files and prints.
I have no experience with the D800/E.