We make the third conditional by using the past perfect after 'if' and then 'would have' and the past participle in the second part of the sentence:
if + sub + had + verb3 + (obj) +, + sub + would have + verb3 + (obj)
It talks about the past. It's used to describe a situation that didn't happen, and to imagine the result of this situation.
-If she had studied, she would have passed the exam.
(but, really we know she didn't study and so she didn't pass)
-If I hadn't eaten so much, I wouldn't have felt sick.
(but I did eat a lot, and so I did feel sick).
-If we had taken a taxi, we wouldn't have missed the plane
-She wouldn't have been tired if she had gone to bed earlier
-She would have become a teacher if she had gone to university
-He would have been on time for the interview if he had left the house at nine.