An insult. Saying someone has 3 eyes expresses offence at an action and/or phrase that they may have just said. They will probably feel embarrassed and ashamed at this, and too right! They deserve to.
"You've got 3 eyes!" **points at persons head with index, middle and ring finger in a sort of triangle**
By Crissie
The Hills Have Eyes 3
A reference to the recently remade movies about radioactive mutants.
It is used as a comment to describe one ugly person or several gross backwards people in a neighborhood or town.