Question

Look carefully at the three prophecies. in what ways does each encourage a sense of false security in macbeth?

Answer 1

All three prophecies seem to make it look like Macbeth will never be defeated, but Macduff and the armies find a way around all of them. 

The first prophecy warns Macbeth to "beware Macduff", but then the second prophecy seems to contradict that by saying  "none of woman born
Shall harm Macbeth.". Macbeth takes this to mean no person can harm him, so why should he be afraid of Macduff. As Macduff reveals in the last scene though, he was not naturally born, he was "ripped" early from his mother's womb. 

The third prophecy seems the most impossible to Macbeth, that he should not fear "until Great Birnam wood to high Dunsinane hill Shall come against him." Macbeth believes it is impossible for the forest to move, but the armies cut down branches and to hold in front of themselves so they can hide their numbers. This makes it appear like the forest is moving towards Macbeth's castle.