Why is the last line of act 1 scene 1 of macbeth said by all of the witches?

The three witches at the beginning of Macbeth tell him that he'll become the Thane of Glams, Thane of Cawdor and the king giving us a clue of upcoming events in the story. In Shakespeare's time some people believed witches were valid sources of information who were able to know how the future was going to turn out and also having the power to change it.