Why does macbeth go to see the witches in this act

Macbeth goes to see the witches again because everything they have told him has been true, so going to see the witches again will help determine whether Fleance is a threat to his throne. 

Things that have been true that he was told:

He became the Thane of Cawdor.
Then became the King of Scotland.

You need to provide more information because i do not know what story it is or character this is referring to.