Insult for someone wearing a monocle.
British guy after tipping his top hat and putting a monocle up to his eye: Pip pip. Cheerio.
American guy: Fuck you 3 eyes!
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**