The Three Concealments
The Head of Bran Fendigaid, ap Llyr, which was buried in the White Hill in London. and as long as the Head was there in that position, no oppression would ever come to this island
The second: the Bones of Gwerhefyr Fendigaid which were buried in the chief ports of this island
The third: the Dragons which Lludd ap Beli buried in Dinas Emrys in Eryri.
The Three Disclosures
The Bones (for the love of a woman)
The Dragons by Gwrtheyrn the Thin
The Head by Arthur because it did not seem right to him that this island should be defended by the strength of anyone other than him.
One of The Welsh Triads
Gwrtheyrn the Thin is of course Vortigern