it really wouldn’t matter because having a 3rd eye would cause an overlap in our visual field, meaing that what your left eye can see is also seen by your 3rd eye. In the condition of being hyperopic or far-sighted when looking a far distance. If we were to look at a person from a close up, there would be two of them due to overlap (it’s like pushing your two eyes together to look at the tip of your nose, only your nose is focused, everything else is out of focus), however for myopic or near-sighted and looking at a farther distance, it might give us greater accuracy however still overlap at a certain distance angle, and amount of light. Basically with a 3rd eye, it would disrupt our perception of the world. Our retinas perceive the world at an inverse image and our brains flip that image over. with a third eye, we’ll lose the balance of just allowing one eye to cover one region, if we use all three, eventually one would be outworked by another due to the overlap and we won’t need a third eye. Evolution is quite interesting…
Our eyes provide a good view of the area directly in front of us, and moderate peripheral vision to the sides. An eye in the middle of the forehead wouldn’t improve on what we’ve got- now an eye above each ear would significantly improve peripheral vision.
FYI predators (lions, wolves) usually have eyes oriented to provide good vision of the prey they are chasing down while prey animals (deer, antelope) have eyes positioned to provide better peripheral vision so they can see what’s coming at them.
I imagine it would be like the third, or parietal eye that some primitive reptiles have. The Tuatara has the most developed parietal eye, with a lens and retina, but it still isn’t as well developed as it’s primary eyes, and remains hidden under translucent scales. It’s also present in some species of fish.
The pineal gland and parietal eye are derived from similar embryonic tissue, so there is likely some sort of evolutionary link between the two (though one may not have directly evolved from the other). In humans, the pineal is involved in regulating circadian rhythms, or rest/activity cycles based on the light outside, among other things. Humans use visual cues that are indirectly translated to the pineal gland, but primitive reptiles use thier parietal eye to pick up these cues directly.
Since the parietal eye uses a different form of photoreceptors than regular eyes, I’m not sure if it would improve vision. If anything, it would probably allow us to make a better guess at what time it is, based on light levels, and could give us an instinctive internal calendar, based on the length of the day.
Kind of a lame skill, if you ask me, but still a neat concept.