WordNet records no synonym and no antonym for triplicate. 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.
one of three copies; any of three things that correspond to one another exactly
reproduce threefold
“triplicate the letter for the committee”
Synonyms, antonyms and the broader and narrower relations for triplicate come from Princeton University’s WordNet. Full sources and licences.