There are two physical eyes, we all can see. There is not a third eye physiologically in your body, it is a metaphor. When you look into existence with absolute undivided consciousness, then you are one. When Jesus said to his disciples, "If your become of one eye, then you will know my Kingdom of God. If you attain one eye, then all bliss be yours and all benediction". He is talking about the third eye.
Two points must be understood; First, being energy of the third eye is really same as two physical eyes. The third eye is non-functioning, it cannot see unless the physical eyes become unseeing. The same energy must move into third eye. It needs energy to function, and the same energy has to be redirected.
Secondly, when you are seeing through physical eyes, you are seeing through physical body. The third eye is not part of physical body; but part of the subtle body (suksham sharir). That's why physiology we cannot believe that there is an existence of the third eye.
With the third eye functioning, you can enter a different dimension. With the third eye you can see things which are invisible to the physical eye but visible to the subtle eye. With the third eye functioning, you look at a person, and you look at the person's soul, at the spirit, not at physical body through physical eyes.
The movement through third eye transforms into a new world, a subtle world. You start seeing things you have never seen.
The third eye is the conscience, the two physical eyes see the past and present, while the third eye reveals the insight of the future.