Answer:
The third murderer in Macbeth is a character who is only referred to as the "Third Murderer." He is not given a name or any significant dialogue in the play.
In Act 2, Scene 2, the First Murderer tells Macbeth that he and the Second Murderer have been sent to kill Banquo and Fleance. He says:
"We are the servants of a great king,
My master and your master." (2.2.17-18)
Later, in Act 3, Scene 1, the Third Murderer is mentioned when the First Murderer tells Macbeth that they have killed Banquo, but Fleance has escaped:
"We have dispatch'd him. Fleance is 'scaped." (3.1.3)
The Third Murderer is not mentioned again in the play, and his role in the plot is relatively minor.