Three subjects more or less connected formed into one continuous poem or subject: thus the Creation, Redemption, and Resurrection would form a triad. The conquest of England by the Romans, Saxons, and Normans would form a triad. Alexander the Great, Julius Caesar, and Napoleon Bonaparte would form a triad. So would Law, Physic, and Divinity. The Welsh triads are collections of historic facts, mythological traditions, moral maxims, or rules of poetry disposed in groups of three.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894