In Macbeth, if the witches have control over the apparitions, then what is the point of the first apparition contradicting the other two?

As it turns out, the other two apparitions (and, for that matter, the circle of kings that later appear) do not actually contradict the first one. The important thing, though, is that Macbeth ignores the first apparition, which warns him to beware Macduff, once he sees the second one, which tells him that no man "of woman born" can harm him. We find out at the end of the play that Macduff was in fact "untimely ripp'd" from his mother's womb (i.e., he was born by Caesarian section) and so the prophecies turn out to be true. But they are certainly misleading, and Macbeth in his ambition and his trust in the witches, who had been unambiguously correct about his future up to that point, acts with confidence. He proceeds to try to kill Macduff, but his assassins only manage to kill the thane's family. The apparitions are thus crucial to the plot--they raise the question, still debated by scholars and fans of the play, of whether Macbeth was more undone by his own ambition or by meddling of malevolent supernatural forces. At the end of the play, confronting his own death, Macbeth asserts the latter.