The three witches -- also known as the weird sisters -- danced and chanted while making evil brew. They appear in four scenes in the play.
They open the play (Act I, scene 1), when they predict that they will encounter Macbeth after the battle's done.
In Act I, scene 3, they give prophesies to Macbeth and Banquo that set the action of the play in motion. In Macbeth's case, they predict that he will become Thane of Cawdor (which happens almost immediately) and King (which happens only because his ambition leads him to commit murder). They also predict that Banquo will be lower ranked that Macbeth but happier, and that his sons will reach the throne.
In Act III, scene 5, Hecate (the goddess of witchcraft) tells the three witches that they shouldn't have acted without her, that Macbeth cares only for his own ends, and that they will prepare a spell to confuse and mislead Macbeth.
Finally, in Act IV, scene 1, they make their famous brew. One of the most famous lines from Shakespeare is found in this scene:
Double, double, toil and trouble;
Fire burn and cauldron bubble.
Then then proceed to conjure up three apparitions that fortell Macbeth's future -- confusingly, but in truth.
The three witches were concocting something foul in Act 5, scene 3. The first witch describes the ingredients:
Then the second witch describes more:
Fillet of a fenny snake,
In the cauldron boil and bake;
Eye of newt and toe of frog,
Wool of bat and tongue of dog,
Adder's fork and blind-worm's sting,
Lizard's leg and owlet's wing,
For a charm of powerful trouble,
Like a hell-broth boil and bubble.
And they add on to that in the scene. Then Hecate enters and the stage directions say "music and song." This means they begins singing and dancing around the cauldron. Then the famous line "By the pricking of my thumbs, something wicked this way comes." That wicked something would be Macbeth. He wants to know more about his future.