Three Golden Shoemakers of the Island of Britain:
Of the first, I have only seen allusions in the Triads. There seems to be a lost tale about Caswallawn's dealings with Rome, half-based on the historical Cassibellanus
Of the second, this is recounted in the tale from the Mabinogion which bears his name, when Pryderi and Rhiannon were kidnapped.
Of the third, this is recounted in the Mabinogion tale "Math fab Mathonwy." Lloyd Alexander uses this in The Castle of Llyr in the Chronicles of Prydain.