I think that Shakespeare decided to include the three witches in his play because when Shakespeare wrote the play, it was in a time where the idea of magic and witchcraft was a large myth. The era Shakespeare was writing in was inclusive of magic and didn’t focus on Christianity. At the time there was also a uniting of the crowns, Which is something that could be represented by the witches. So the main reason Shakespeare included the Witches in his work is because it is a part of the era. He was born a year after the Witchcraft and Conjuration Act of 1563, meaning that anyone will be “killed or destroyed” by use of witchcraft punishable by death.