What role does Ross play in Act 4, scene 3, of Macbeth?
Although Ross does not appear for very long in this scene, the part he plays is extremely important. In this scene he is a messenger, and he brings Macduff the grim news that Macduff's family has been slain by Macbeth. This is what impels Macduff - with encouragement from Malcolm - to an act of revenge against Macbeth, which will form the climax of the play.
Therefore, although overall Ross is one of the more minor characters, and his appearance in this particular scene is relatively brief, he helps set in motion a chain of dramatic and decisive events, which culminate in the final defeat of Macbeth.