Bendigeidfran's head was one of the Three Fortunate concealments; what are the other two?

Question

Bendigeidfran's head was one of the Three Fortunate concealments (emphasis mine):

'Shame on my beard,' said he 'if I don't open the door and find out whether it is true what is said about it. [So] he opened the door, and looked out to Cornwall and over Aber Henvelen. And when he looked, suddenly everything they had ever lost - loved ones and companions, and all the bad things that had ever happened to them; and most of all the loss of their king - became as clear as if it had been rushing in towards them. And from that moment, they were not able to rest unless they were making for London with the head. However long they were on the road, they came to London, and they buried the head in The White Hill.

And that was one of the Three Fortunate concealments when it was buried, and one of the Three Unfortunate Disclosures when it was unearthed: since no affliction would ever came to this Island from across the sea, as long as the head was in that concealment. That is what this tradition says. Their adventure 'The men who set out to Ireland', is [the name of] that [tale].

_{Source: The Mabinogi of Branwen}

What were the other two Fortunate Concealments? What was their significance, and in what stories do they appear?

Answer 1

1. The dragons trapped by Lludd.

   In The Adventure of Lludd and Llefelys, there are three plagues which fall upon Britain. The second is a horrible screech that renders the inhabitants scared. King Lludd, on the advice of his brother, Llefelys, builds a pit in the center of Britain, at Oxford. He traps the dragons responsible for the screeches by getting them drunk with mead.

   This dragon is being attacked by a foreign enemy. The burial of the two dragons keeps Britain safe from invasion (making it one of the Three Concealments, until they are revealed again by Vortigern (one of the Three Disclosures.

2. The Bones of Gwerhefyr Fendigaid.

   The other two are the Bones of Gwerhefyr Fendigaid, buried in various ‘great ports’ around the coast, and the Dragons which Lludd ap Beli or Lugh of the Silver Hand, Celtic god of health and healing, captured at Oxford – center of the ancient kingdom – and buried in Dinas Emrys in Snowdonia, deemed to be the strongest part of the island.

   The Concealments are also detailed here, in what seems to be an original manuscript, The Welsh Triads, from the 14th century:

   The Head of Bran the Blessed, son of 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 Gwerthefyr the Blessed, which were buried in the Chief Ports of this Island;
   The third: the Dragons which Lludd son of Beli buried in Dinas Emrys in Eryri.

   This text also details the Three Unfortunate Disclosures, which are based on the Three Unfortunate Concealments:

   And there were the Three Unfortunate Disclosures when these were disclosed. And Gwrtheyrn the Thin disclosed the bones of Gwerthefyr the Blessed form the love of a woman: that was Ronnwen the pagan woman;
   And it was he who disclosed the Dragons;
   And Arthur disclosed the head of Bran the Blessed from the White Hill, because it did not seem right to him that this Island should be defended by the strenght of anyone, but by his own.