based on my presumption in the third murderer in Macbeth
the third murder would be Macbeth servant
before and after Marybeth's soliloquy in lines 45 and in between 71-72
its shows that the servant knows that banquo is attending the party
and the servant knows whats happening in the palace gate(line 46)
when the murder happened (scene 3 line 2) the first murderer ask who send the third murderer in the murder of banquo and the third murderer said macbeth
in addition in line 13 the third murderer said that "almost a mile;but he does usually,make it their walk."
this supports that (back in line 46) the servant knows what the activities happening in the palace gate.
and also the last person whom macbeth have seen and spoken, was the servant.(line46 and after the line 72)
so i think the servant is the third murderer