This is the 3687th most frequent Russian word.
третью
"Третью" means "third" (feminine instrumental case) in English.
The word 'третью' here is used in the accusative case (feminine singular) to indicate an ordinal numeral describing the noun 'книгу' (book) as the third item.
Я читаю уже третью книгу на этой неделе.
I am already reading the third book this week.
In this sentence, 'третью' is in the accusative case (feminine singular) and refers to 'дорогу' (path) as the third option being chosen.
После долгих раздумий я выбрал третью дорогу.
After long contemplation, I chose the third path.
Here, 'третью' is used in the accusative case (feminine singular) to describe 'работу' (work or paper) as the third item being reviewed.