Why do the two murders trust the third

The presence of a third murderer and the man's identity has been a source of debate for year.... All we know is that the third man claims to have been sent by Macbeth, we also know the first two murderers comment on the fact "he" could have trusted them. Thus, we can infer they accepted the man's presence because Macbeth sent him..... whether or not they truly trusted him is unknown.