third has 7 distinct synonyms spread across 6 of its 8 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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 equal parts of a divisible whole
“it contains approximately a third of the minimum daily requirement”
the fielding position of the player on a baseball team who is stationed near the third of the bases in the infield (counting counterclockwise from home plate)
“he is playing third”
Synonyms, antonyms and the broader and narrower relations for third come from Princeton University’s WordNet. Full sources and licences.
following the second position in an ordering or series
“a distant third”
the musical interval between one note and another three notes away from it
“a simple harmony written in major thirds”
the third from the lowest forward ratio gear in the gear box of a motor vehicle
“you shouldn't try to start in third gear”
the base that must be touched third by a base runner in baseball
“he was cut down on a close play at third”