first
middle
final, last
the last third of the novel

remaining
bottom, lower
top, upper
Most of the students' scores were in the upper third of the range.

divide sth into
He divided the money into thirds.

third of
Over a third of sales were made over the Internet.

about a third, almost a third, at least a third, over a third
Over a third of people questioned said they would consider cosmetic surgery.

between a third and a half, a third to (a) half
The discount is a third to a half of the full price.

Third is used with these nouns:
world