Macbeth-Who is the third murderer who appears in Act III, scene iii?

Write an analysis of the identity of the third murderer in Macbeth and why he appears in Act III, scene iii. With close reference to the text, you need to support your choice as to whom you believe Shakespeare intended that extra person to be. Through carefully reading the text, I believe it is up to the individual's imagination to determine who the third murderer is. Shakespeare is not clear on this issue and I …

…escorts the two murderers into Macbeth's presence (Act III, scene i, between line 73 and 74), was right outside the door during this time (Act III, scene i, between line 74 and 75), and as is his duty, is always attending to Macbeth, and therefore must spend a lot of time. Thus, Macbeth must have come to trust this attendant, and gotten him to go ras the third murderer and join the other two murderers in their pledged deed.