noun
Triple Crown horse
Specifically refers to a horse that has won Japan's three main classic races (Satsuki Sho, Tokyo Yushun, Kikuka Sho).
ディープインパクトは日本競馬史上6頭目の三冠馬となった。
Deep Impact became the sixth Triple Crown horse in Japanese horse racing history.
三冠 refers to the three titles or the achievement itself, while 三冠馬 specifically means the horse that achieved it.
Compound of 三 (san, 'three'), 冠 (kan, 'crown'), and 馬 (ba, 'horse').