The first apparition is described as an Armed Head. It tells Macbeth to beware Macduff, the Thane of Fife, and nothing more. The second apparition is described as a Bloody Child. This is appropriate since this apparition tells Macbeth that none of woman born shall harm him. The bloody child must symbolize a child that has been taken out of its mother's womb via a very primitive C-section. The third apparition is described as a Child Crowned, with a tree in his hand. This apparition assures Macbeth that he can never be vanquished until Birnam Wood shall come against him on Dunsinane Hill. The crowned child represents Banquo's descendant, and the tree represents Birnam Wood.
The first apparition causes Macbeth to decide to have Macduff killed. But Macduff has already fled to England, and Macbeth's soldiers only succeed in killing his wife and children. This terrible deed only makes Macduff more determined to get revenge, and he eventually kills Macbeth on the battlefield. It also creates more sympapthy for Macduff and more hatred for Macbeth.
The other two apparitions fill Macbeth with false hopes and reckless courage. He becomes more and more bloody and tyrannical until the whole country is turned against him. When the English forces approach, it is augmented with many Scots who have fled to join them.
It is significant that Macbeth says, "Then live, Macduff; what need I fear of thee? / But yet I'll make assurance double sure / And take a bond of fate." Taking a bond of fate is a beautiful metaphor. Throughout the play Macbeth seems to be fighting against fate more than against any human. It might be said that the main conflict in the play is not man versus man but man versus fate. This is obviously an impossible task, but it does make Macbeth seem more heroic. He is somewhat like Prometheus and one or two other characters from mythology who have defied the gods. Orpheus was another.