next after the second; being the ordinal number for three.
being one of three equal parts.
Automotive. of, relating to, or operating at the gear transmission ratio at which the drive shaft speed is greater than that of second gear for a given engine crankshaft speed, but not as great as that of fourth gear, if such exists: third gear.
rated, graded, or ranked one level below the second: He's third engineer on the ship.