The reptilian brain visual system is a lot simpler. Snakes do not see in stereo as the eyes are on opposite sides of the head. Even this guy has them at about 90 degrees, so there is probably little overlap. This means there is no need to coordinate both eyes together the way we do. Chameleons do use stereo vision, but they do it differently - they target an insect with one eye, then bring in the second one to confirm depth just before firing the tongue. So, maybe a third eye would work fine on a snake, but not on a mammal because of the extra wiring we have.
The reptile brain does have a sort of LGN, which is where a lot of the fusion of our two eye images happens in our heads. Maybe something with three eyes needs three of these, or maybe six for full trinocular vision. But the third eye is looking up, so it is hard to guess what information it could fuse with the other views.
Nice try, evolution, but I don’t think it will catch on.
