삼수가 · 삼수를
third attempt at the college entrance exam (after failing twice)
Example
그는 삼수 끝에 원하는 대학에 합격했어요.
He got into the college he wanted after his third try at the entrance exam.
Word parts
삼 three+수 study
Literally 'third study'; the second attempt is 재수.
Good to know
Also used as a verb: 삼수하다, 'to take the entrance exam a third time'.