WordNet records no synonym and no antonym for triple crown. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
(horse racing) a title won by a horse that can win the Kentucky Derby and the Belmont Stakes and the Preakness
(baseball) an unofficial title won by a batter who leads the league in hitting average, runs batted in, and home runs
Synonyms, antonyms and the broader and narrower relations for triple crown come from Princeton University’s WordNet. Full sources and licences.