1. 特指一个物品排序,the third
譬如:我要第三个杯子。 I want the third cup.
2. 比赛排第三, the third place
譬如:杰克在跑步比赛中获得了第三名。Jack won third place in the race. Jack was third place in the race.
3. 三分之一(如一楼的)a third / one thirds
譬如:我们班里面三分之一的人是女生。One-thirds of the people in our class are girls. A third of the people in my class are girls.