Just bought 5D mark III - heard Nikon D800E is much better image Quality? did i make a mistake?

In a word: "no." You didn't make a mistake.

Quite a few people chop in their lenses from camera maker A to camera maker B, take a loss, then lament their change. I beleive you'll be quite pleased with the improved autofocus of the 5D Mark III.

Will there be hair-splitting moments where a D800E may serve you better? Sure. And there will be plenty where the camera in hand will make you money -- stick with it.

--
'Nice pen, bet you write good stories with it.'