Upon loading the product images for the Canon EF 400mm f/2.8L IS III USM Lens, the first side-by-side comparison I wanted to see was of the three 400mm f/2.8 IS versions. I figured you might also want to see them.
At first glance, it appears that little has changed between the II (center) and III (left), but upon closer inspection, it seems that nearly everything has been changed. Hit the last link above to see larger versions of these images, but especially note that the tripod collar and foot have been moved significantly rearward, reflecting the much-improved weight distribution of this much lighter lens.