When they first meet him, they say "Hail Macbeth, Thane of Glamis, Hail Macbeth Thane of Cawdor, Hail Macbeth who Shalt be King Hereafter." Macbeth, at this time, is already thane of Glamis. However, the other two positions are the predictions. Therefore, they predict Macbeth will become Thane of Cawdor and will later become King.
Hope this helped!! :D