THIRD CONDITIONAL

 We use the third conditional to talk about something in the past that did not happen.

How is the third conditional different from the other conditionals?

This is the way we imagine how things could have been different in the past. If something had been different, something else would have happened. 

If she’d known, she would have worn something nicer.

In this case, she didn’t know about the party so she didn’t wear special clothes.

GRAMMAR 

if + past perfect, ...would + have + past participle

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.

PRACTICE 

https://test-english.com/grammar-points/b1/third-conditional-past-unreal-situations/

This video explain better this topic.