The first witch encountered a sailor's wife who had chestnuts on her lap. The witch told the sailor's wife to give it to her, but she refused. The sailor came to her defense, but the witch cursed them and they died.
Aroint thee, Witch!
The first witch is telling the other two witches what she had done to the sailor and his wife. She even shows them the sailor's thumb. The witches hear drums and know Macbeth is coming.
Here I have a pilot's thumb
A drum, a drum! Macbeth doth come.
Macbeth and Banquo enter the scene coming back from a battle. On their way back they encounter three witches.
All hail Macbeth!
Why do you start; and seem to fear things that do sound so fair?
The three witches tell Macbeth a prophecy. Macbeth will become thane of Cawdor and later on he will become king. The witches also tell Banquo that his sons will be kings.
Banquo and Macbeth, all hail!
Two messengers from the king, Ross and Angus, came to tell Macbeth that the king has named him thane of Cawdor. They are shocked that the prophecy of the witches came true. The thane of Cawdor is still alive, but he committed treason.
He bade me from him, call thee thane of Cawdor
What, can the devil speak true?
Macbeth is okay with being named thane of Cawdor and tells Banquo that if he is to become king, he wouldn't mind. Banquo thinks that the whole situation is strange and is worried that something bad might happen.