Three men rule as King of Scotland at different points throughout the play. Initially, King Duncan is the benevolent, just ruler of Scotland, who appoints Macbeth as the new Thane of Cawdor after the former Thane of Cawdor is executed for treachery. Following Macbeth's successful victories against the Irish and Norwegian armies at the beginning of the play, Macbeth and Banquo listen to favorable prophecies from the witches. The Three Witches refer to Macbeth as the Thane of Glamis, Thane of Cawdor, and future King of Scotland. Macbeth allows his ambition to affect his actions and ends up assassinating King Duncan with the help of his wife. After King Duncan's assassination, Macbeth is crowned king, while Malcolm and Donalbain flee from Scotland. Macbeth rules Scotland as a ruthless tyrant, while Malcolm gathers an army in England to help him regain his rightful throne. Towards the end of the play, Malcolm leads his army along with the help of Macduff and Siward to Scotland, where they defeat Macbeth in the final battle. After Macduff decapitates Macbeth, Malcolm is restored to his rightful throne and becomes the King of Scotland.

In Macbeth, there are three men who reign as the King of Scotland.

First of all, there is King Duncan. When the play begins, King Duncan is on the throne and successfully thwarts a rebellion by the Thane of Cawdor and his foreign allies. Duncan rules until he is murdered by Macbeth in act 2, scene 2.

After Duncan, Macbeth becomes the king of Scotland. He is not of royal birth but is able to seize the throne in the aftermath of Duncan's death. His rule is characterized by violence and tyranny and is brought to an end when he is murdered on the battlefield in act 5, scene 8, by Macduff.

With Macbeth's death, the throne passes to its rightful heir Malcolm. As the son of King Duncan, Malcolm is the true successor, and he is finally able to take his place on the throne when Macbeth is killed.

Three...Duncan who is killed by Macbeth, Macbeth who is killed by Malcom. Malcom is reigning when the play ends.