In the play Macbeth how are the three kings (Duncan, Malcolm, and Edward) similar?
It is difficult to make much of a judgement about Edward, who does not have a speaking role in the play. He is only mentioned in the third person, as a king that has been hospitable to Malcolm in his exile. Malcolm also describes Edward's ability to cure the skin disease scrofula with the "royal touch." This was a common folk belief in Shakespeare's own day, and the monarchs of England made a ceremony of laying hands on scrofula sufferers each year. So we might say that the king of England is portrayed as a pious man who fulfils his obligations as monarch. Where Macbeth destroys lives, he uses his power to sustain them.
As for Duncan, there is little doubting that he is a good king and a decent man. Macbeth himself acknowledges as much when he is debating whether to kill the monarch:
Hath borne his faculties so meek, hath been
So clear in his great office, that his virtues
Will plead like angels trumpet-tongued against
The deep damnation of his taking-off...
Duncan also trusts and holds Macbeth in very high regard, which makes Macbeth's treachery all the more horrible. Malcolm is portrayed as a noble figure. While he prudently flees Scotland, we learn on Macduff's visit to him in Edward's court in England that he is committed to avenging his father's murder. At the end of the play, with Macbeth dead, Malcolm is a decisive and strong leader. All three kings, then are portrayed as virtuous and good, a stark contrast with the wicked Macbeth.