The third assassin knows Banquo well, but there is no suggestion provided by Shakespeare to deduce that he may be Macbeth’s servant although the text is clear in suggesting that he is in some way in Macbeth's confidence or employ in a position of trust: "First Murderer: But who did bid thee join with us?--Third Murderer: Macbeth" (III.iii).
Macbeth sends the third assassin entirely to help the other two murderers, and he appears just before the murder is committed. Without him it would have been difficult for the others to identify Banquo and understand Banquo’s general activities. It must be noted in this regard that it is the third murderer who first identifies Banquo: “Hark! I hear horses” (line: 9) and “’Tis he” (line: 14).
Hark! I hear horses.
[Enter Banquo, and Fleance with a torch]
The identity of the Third Murderer in Act III Scene iii is debatable. It is stated that he has been sent by Macbeth, and the text suggests that since he is familiar with Banquo and his habits he consequently has been sent to make sure the two assassins complete their assigned task. The Third Murderer may be one of Macbeth's trusted attendants, as the text may suggest, who has clearly joined the two assassins at Macbeth's request.
Enter three Murderers
But who did bid thee join with us?
Our offices and what we have to do [...]
Then stand with us.
The subject of our watch.