Bromwich, R. (ed. & trans.) (1961; new edition 1978; reprint 1991; new edition 2006) Trioedd Ynys Prydein: The Welsh Triads University of Wales Press, Cardiff.
This site used the 1978 edition initially, but references are slowly being updated to the 2006 edition.
You can find a collation of some of the Triads at Mary Jones' Welsh Literature Site.