The First Apparition: Armed Head. It tells Macbeth to be beware of Macduff, The Thane of Fife. Macbeth already could tell Macduff opposed him, so he thinks he is prepared.
Second Apparition: Bloody Child. It says that no man born of women shall harm him. Now, Macbeth feels like he has nothing to worry about due to lack of wisdom and pride. He will no longer worry about Macduff.
Third Apparition: Child Crowned, with a tree in his hand. It warns him "Macbeth shall never vanquished be until Great Birnam Wood to high Dunsinane Hill...." This is foreshadowing of his downfall, when quite literally, wood from Great Birnam is seen from the Hill as he is being attacked. Macbeth thinks there is no way this could ever happen.