What happens in act 3 to illustrate the truth of the witches predictions?

Analysis

The “be a man” theme recurs in Macbeth’s address to the murderers. When Macbeth demands whether the murderers have the courage to kill Banquo, they answer "we are men, my liege" (III i 92). But their answer does not satisfy Macbeth, who berates them as less-than-exemplary examples of men. Macbeth thus uses very much the same goading tactics his wife used in compelling him to kill Duncan. But what does it mean, exactly, to “be a man”? Both Macbeth and his Lady seem to have a clear idea of properly masculine actions. In Act 1, Lady Macbeth suggests that masculinity is largely a question of ruthlessness: one must be willing to “das[h] the brains out” of one’s own baby (58). She claims that she herself is less "full o' th' milk of human kindness" than Macbeth—that is, more capable of casting away the last shreds of compassion, tenderness, loyalty, and guilt.

Lady Macbeth is not the only character that values ruthlessness as a masculine trait. Duncan, too, evaluates heroic action on a rather gory scale. When the captain describes how Macbeth “unseamed [Macdonald] from the nave to th’ chops” with “his brandished steel / Which smoked of bloody execution,” Duncan responds with high praise: "O valiant cousin, worthy gentleman" (I ii 17-22)! A "real man” in Macbeth, then, is one who is capable of copious bloodshed without remorse. The catch, of course, is that the bloodshed must be justified. Whereas Macbeth needs no reason to slay Macdonald in battle per se, the two murderers require the justification that Banquo is an evil man.