There are many references throughout the stories of The Mabinogion to groups of three, as well as instances where repeating something three times is used as a linguistic/literary device. For example, in ‘The Dream of the Emperor Macsen’, the same journey is described on three separate occasions. It seems that grouping in threes had a special significance in early Welsh Literature, as evidenced by a series of medieval Welsh manuscripts known as 'The Welsh Triads' (Trioedd Ynys Prydein). Many of the examples given in The Mabinogion appear in the Triads.
The Three Chief Maidens is an example of a triad in The Mabinogion. Others include the Three Undemanding Chieftains and the Three Golden Shoemakers (Third Branch); The Three Disloyal Retinues (Fourth Branch), and the Three Adjacent Islands (‘The Dream of the Emperor Maxen').