Why did Macbeth stand amazed at the prediction of the three figures?

Macbeth doesn't believe them at first, but...

1) Thane of Glamis- He was already this so technically it's not a prediction.

2) Thane of Cawdor- He was named this by the king really soon after they gave his the predictions (same day).

3) Because the first 2 were true, he believed he would be king and he killed Duncan to "speed up fate."