What convinces Macbeth that the Witches’ prophecy is true?

Macbeth becomes convinced that the Witches’ prophecy is true when Duncan names him Thane of Cawdor, which the Witches prophesied would happen. When the three Witches first approach Macbeth, they acknowledge Macbeth as Thane of Glamis (his current title) as well as Thane of Cawdor. This puzzles Macbeth since he can’t figure out how he is both. Shortly after, Ross delivers the news that the king has given Macbeth the new title of Thane of Cawdor, since the previous Thane of Cawdor has been executed for treason. This unexpected event causes Macbeth to become convinced that the Witches were telling the truth.