Interesting as according to DXO Mark, the camera with the greatest DR of the three [Canon ones] you mention is the 5DSR. It also scores higher in colour depth.
A friend had a Canon 5DSR and well honestly it was terrible, unless it was pinned to a tripod it was hard to get good sharp images, it was fussy with lenses and the DR was very poor. He just couldn't live with it and traded it for a 1DX (he also had a 5DIII).