Third Conditional

Type III – expresses regret to something you have or haven't done; impossible, imaginary situations.


Strucures: 

If + Past Perfect + would have + Past participle or Would have + past participle + if + past perfect


Examples:

a. If Gonçalo  had studied harder, he wouldn’t have failed the exam.

b. Alexandre would have phoned Rafael if he had known he was in trouble. 

Have a look at these links:
http://www.ego4u.com/en/cram-up/grammar/conditional-sentences

https://www.englishgrammar.org/conditional-grammar-exercise/