It has nothing to do with a third eye. It’s recorded numerous times throughout the bible.
Keeping the commandments of God is a sign on the forehead and mark of the believer. In Revelation, the Father sends out angels to mark the forehead of his elect ones. This is also spoken of by the prophet Ezekiel.
The forehead represents ones thoughts or belief and the hand represents ones actions. The name represents authority that we are submitted to, expressed through our thoughts and actions.
Either to the beast or the Creator. Either following the commandments of the Most High through faith in the Messiah or a life of sin and lawlessness.
Except that John wrote the book of Revelation in Greek and didn’t write “666”.
Some translations have “616”, other “six hundred six fold and six” but these are all just translations and translations require the author discernment.
In Greek, John wrote “chi-xi-stigma”. Go down that rabbit hole.
But, remember that he also said “let he who hath wisdom....”. Scripture also tells us “a fool says in his heart ‘there is no God’” and that “the fear of God is the beginning of wisdom”.
Unless you are aligned with the faith and Truth and wisdom of scripture, you will never have understanding on this.
For example, most people studying this are still unaware of the name of the Father, or what “name” means in biblical references (not only what we call someone but their character, power and authority-particularly in our lives) or that the Father puts his name, his mark, on the forehead and right hand of his faithful elect ones.
Once you understand the mark of the Father, you can come to understand better the counterfeit mark of the beast.
I just posted about this recently, read my post for details on what happened.