Having 3 Eyes? Watch
Why didn't human evolve to have 3 eyes? It may sound like a ridiculous but an eye at the back of the head when position correctly can give us a full 360 degree vision. That would help both hunting and watching out for predators.
The predator argument isn't a great one. We don't have many predators, at all. And for prey, as omnivores its not essential we can hunt successfully as we grow plants to eat too. In animals that do have lots of predators, their eyes are on the sides of their heads-like in cows, giving them almost 360 degree vision.
There also arent many organs we have 3 of. We have two hands, legs, arms, feet. During development, a big reason for this is probably in the establishment of the Left-right axis. Our bodies produce sonic hedgehog protein (Shh), and cilia on our cells moves it more to the left hand side of the body, establishing internally which side of the body is the left. Presumeably a third eye would end up in the centre of our head, on either side (front or back), so it would be difficult to have cells sensitive to intermediate levels of Shh.
Additionally theres the issue of innervating it-our optic nerve leaves the back of the eye and heads straight to the occipital lobe, which is a thin sliver of brain cortex at the back of your head. If an eye was in the back of your head, it would be positioned inside the occpital lobe-you would respond to stimuli from that eye before the front of head eyes. That could make life interesting
My final thoughts...are on visual fields. Our visual fields of each eye we currently have overlap, and our brain processes and corrects for this in the occipital lobe in processing. With three eyes-this would need substantially more correction, if all three eyes faced forward, OR substantially more processing power in your brain, to cognitively process inputs from two directions, and produce appropriate responses to each as required.
Plenty of thoughts there, but thats my thoughts, as a developmental biologist with a bit of interest in Evo-Devo.