In the third branch of the Mabinogion, Pryderi tells Manawydan

Your cousin is king of Island of the Mighty and although he may do you wrong' he continued 'you have never been a claimant for land or territory. You are the third Humbled Chieftain.'

Going through the footnotes, I read that the other two Humbled Chieftains were Llwarch Hen and Gwgon Gwron. It would seem, from Pryderi's remark, that there is some connection between the three, yet the other two do not (if I recall correctly) appear in the Mabinogion.

Is there a connection between the Three Humbled Chieftains?