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.