What will happen when the three men meet face to face, or force to force, is what does happen when they meet - Macduff kills Macbeth. When Macduff first hears about what Macbeth did to his family, he is grief-stricken. Malcolm tells him to turn his anger toward Macbeth and sharpen that anger. He tells Macduff not to let grief make him weak; grief should make him stronger and more determined. That is exactly what happens. When Macduff and Macbeth meet in the last scene of Act 5, Macbeth still thinks he is invulnerable because "no man born of woman" can harm him. He doesn't want to fight Macduff because he feels he's caused enough bloodshed in Macduff's family already. Macduff tells Macbeth that he will fight and that he was not born in the traditional way, therefore is outside the category "born of woman". Macbeth and Malcolm do not meet up face to face, but Malcolm's forces meet up with Macbeth's forces and Malcolm's troops defeat Macbeth's. At this point, Macbeth has become despondent - his wife is dead, his men are defeated, his country does not want him as its king. It is very possible that he is not fighting to his full abilities whereas Malcolm and Macduff are fighting with full effort because they have revenge driving them.