Within Christianity, the foundations of religion in Christian society, there is a holy trinity which is the idea that God exists in three different forms: The Father, Son, and the Holy Spirit. As the witches always appear as a trio, they could be seen as the anti - trinity, as they persistently tempt fate and indulge in wicked acts.
Shakespeare intends to be condemning of the Witches and their wickedness, as this would directly appease James I due to him believing they were agents of the devil, which is seen in 'Daemonologie'.