The third conditional talks about a possible event in the past that did not happen, and it is now impossible for it to happen.
- If it had rained yesterday, I would have stayed at home.
(event didn’t happen/it didn’t rain – result didn’t happen because the event didn’t happen/didn’t stay at home)
The third conditional is formed if + past perfect + would have + past participle
- If I had seen her, I would have told her.
(if + past perfect/had seen + would have + past participle/told)