This conditional expresses a situation in the past that did not happen, so when we use this conditional we are actually imagining a different past that cannot be changed. This unreal situation can never happen, not now or ever.
With the Type 3 Conditional, we use the past perfect in the if-clause and the perfect conditional (would have + past participle) in the main clause to:
1. express disappointment or regret about things that are not possible to happen now.
- If you had married his brother instead of him, you could have been a happier person.
- If you had taken up teaching, you would have been better off.
2. talk about things that should have happened in the past but didn’t.
- If he had gone to the doctor earlier, he wouldn’t have got this disability.
- If I had completed my education, I would have been a person of importance or authority.
3. imagine unrealistic situations.
- If I had been taller, I would have been a professional basketball player.
- If I had been a businessman, I wouldn’t have led a struggling life.
4. express criticism.
- If he had got up earlier, he wouldn’t have missed the bus.
- If you had been a bit more patient, you would have got what you wanted.