Actually, Macbeth does not appear in Act III, Scene v. I am sure you must mean Act III, Scene iv. It is in this scene that Banquo's ghost appears at the dinner Macbeth and Lady Macbeth are holding for their guests. Of all in the room, only Macbeth sees the ghost.

Banquo's ghost first enters the banquet hall (without Macbeth's having seen him) and sits in Macbeth's place at the table. When Macbeth realizes that the ghost is sitting in his chair, Macbeth speaks to him, denying immediately that he played any role in Banquo's death--which is a lie, of course. Macbeth continues to speak to the ghost, while Lady Macbeth tries both to get him under control and to explain his strange behavior to their guests. As the scene continues, the ghost leaves, and things return to some semblance of normality. Then the ghost returns, and Macbeth again addresses him, this time violently. The ghost leaves again, for the last time, and Lady Macbeth dismisses the dinner guests.

Macbeth had been horrified by the ghost of Banquo. It had appeared to him still bloody from Banquo's murder. The fact that only Macbeth sees the ghost reveals his guilt in Banquo's murder. Of all in the room, only Macbeth is responsible for the cruel and violent murder of his former friend; he is the only one who deserves to be haunted by this terrible deed.