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 have a few theories on why the third murderer appears in Act III, scene iii.
My first theory is that there is a third murderer because throughout the whole play, everything is d…