Three murderers in ACT 3 SCENE 3

In act 3 scene 3 first only 2 murderers appear according to Macbeth which have invited them to kill Banquo then afterwards in the scene where Banquo and Fleance appears- the 2 murderers but so called the 3rd murderer appears in the scene!

Who do you think the "third murderer" would be: Macbeth or someone else???

Any reasons whether it is or not Macbeth??