Lizards have a little dot at the top of their heads that is called a "parietal eye." This eye is not as complex or as useful as the two in the front of their heads, but it does react to light. Are they eventually going to have a third eye? No. It's just a vestigial trait.
In fact, it looks like most animals — including humans — had a chance at a third eye, and we blew it.
Top image: 3x3 Eyes.
The tuatara is an endangered species, and lives on only a few islands in New Zealand, each of which is carefully maintained as a native animal preserve. It looks like a lizard, but isn't one. It's leftover from a time 200 million years ago, when tetrapods were turning into turtles, lizards, crocodiles, and dinosaurs. It hasn't changed since then, and so represents a look at what animals of that particular time period were like.
And it has a spot on the top of its head, which the closest thing anybody has to a third eye.
An extra eye presents a lot of evolutionary benefits. Being able to look up, or just behind you, for predators seems like an advantage to nearly any species. Although many lizards also have this spot, it has been lost in turtles, crocodiles, and birds. By examining the physiology and development of these species, from humans all the way back to the well-developed parietal eye of the tuatara, scientists are figuring out how later species lost a third eye, and what we might have gained in its stead.
Perhaps the most notable feature of the third eye is that it's not symmetrical. Drop a line down the center of a body, even a developing body, and it's almost certain that the left and the right side will match. That's how regular eyes develop. They start off as dents in the developing roundness of the head. As the dents grow inwards, the finer structures of the eye develop. The parietal eye doesn't dip inwards. Instead, the outside of the whole structure is lined and becomes a kind of a bump, and inside two symmetrical parts of the brain structure develop. The left side of the brain becomes the parietal eye. The right side becomes the pineal sac. In reptiles, the parietal eye takes in light, and the pineal sac puts out melatonin, a hormone that regulates sleep cycles.
In humans, instead of the pineal sac wandering up towards an eye on the top of our heads, we have the pineal gland which stays down near the rest of our brain. The pineal gland also puts out melatonin, and is studied in sleep research. It also puts out many other hormones which focus on neurological regulation — the most famous of which is serotonin. The proper functioning of the pineal gland keeps people focused, happy, awake in the day and asleep at night, and prevents neuro-degeneration as people age. It seems that even if we could have a third eye, it would be at the expense of being able to sleep, be happy, and keep mentally fit.
It would also be at the expense of our hair.
Tuatara Image: Phllip C