In Act III, Scene 1, Macbeth convinces two apparently ordinary commoners that Banquo has been their enemy and that they should kill him.
That it was he, in the times past, which held you
So under fortune, which you thought had been
Our innocent self?
Evidently it really was Macbeth who, for whatever reason, had held them "so under fortune," but he succeeds in getting them to believe it was really Banquo. Shakespeare may have felt it was credible for Macbeth to do such persuading to two men but that if he added a third man it would seem less so. But the playwright must have realized that he would need three men to ambush Banquo and his son. Banquo is a mighty warrior. It would take at least two men to kill him. But he would need a third man to attack Fleance. Shakespeare simply had the third murderer show up at the ambush scene and tell the other two that Macbeth had sent him. The Third Murderer does not necessarily hold the same grudge against Banquo as the other two. This Third Murderer might be just a paid assassin or might have been stirred up against Banquo with some other pack of lies.
Shakespeare must have felt that the meeting between Macbeth and the first two murderers would be easier to handle and Macbeth's inciting lies more effective with two men than with three. When the Third Murderer joins the other two in Act III, Scene 3, he is used to provide information to them, and to the audience, that had not been provided before.
His horses go about.
Almost a mile, but he does usually—
So all men do—from hence to the palace gate
Make it their walk.
Banquo and Fleance have been out riding all day. But they could hardly appear on stage on horseback! So Shakespeare has to explain why the father and son are on foot. It seems odd that visitors to the palace would leave their horses almost a whole mile from the palace and then approach on foot. But Shakespeare had to have Banquo and Fleance on foot when they are attacked. Even if Shakespeare could have shown father and son on horses, that would have made it very difficult for the three murderers to attack them--but showing them on horseback on an Elizabethan stage was out of the question anyway.
If the first two murderers already knew that Banquo and Fleance would be on foot, then there would be no excuse for having them explain it to each other. But evidently they don't know this fact. What the Third Murderer tells them is intended to convey this information to the audience, who would otherwise wonder why the father and son are not riding their horses when they are, of necessity, so far away from the palace. Macbeth would have wanted the ambush to take place at least a mile from his palace because he doesn't want any hint that he might have been involved in the plot. As he tells the two murderers in Act III, Scene 1:
...and though I could
With barefaced power sweep him from my sight
And bid my will avouch it, yet I must not,
For certain friends that are both his and mine,
Whose loves I may not drop, but wail his fall
Who I myself struck down. And thence it is
That I to your assistance do make love,
Masking the business from the common eye
For sundry weighty reasons.