How has Macbeth changed between the beginning of Act I and the end of Act III?
In the first two scenes of Act I, the only thing we know about Macbeth is that he has fought bravely in battle and appears to have been a loyal subject to the king. In Scene III, he is startled to hear the witches say he is the Thane of Cawdor and will be king. He can not even believe that he would be king. ". . . and to be king / Stands not within the prospect of belief." (I.iii.76) When Macbeth learns that he has been given the title of Thane of Cawdor, he only begins to think about being king. This marks a turn in his thinking. At the end of this scene, he decides to wait and see what happens.
In Act III, Scene IV, Macbeth has become a tyrant. He greets the murderers who have returned from killing Banquo. Then at a dinner, he sees Banquo's ghost. He has become a tyrant with a guilty conscience. He is so anxious that he is hallucinating. He feels guilt but also fear of being found out or being dethroned. In the beginning of Act I, Macbeth was still a loyal subject, a simple man with a simple consciousness. But by the end of Act III, he has killed Duncan, his guards, and Banquo. He has digressed from that loyal, simple, subject to a tyrant with mental burdens of guilt and fear inspired by greed and anxiety.