If I had a third eye, I’d like to have it on the back of my neck. That way I could keep an eye on people that are trying to sneak up on me. I’d say to them, “Hey Mr. Sneaking up on me guy! I can see you with my third eye”. They would then run away due to the fact that the element of surprise is now gone. That would rule.
I’d also use it to look at my tail and the fifth testicle I have hidden on the back of my knee.