The Three Tribal Thrones of the Island of Britain:
Mynyw is modern St. David's. Dewi is St. David/St. Dewi/St. Dafyd (I've seen it spelled all three ways).
Caradawg Strong-Arm is the same Caradoc of French Romance.
The earliest fragments of the triads do not contain this triad; it is thought to have been added by a scribe in 1150 or so, in order to claim the importance of St. David's bishopric.