· Conditional Sentences Type 3
→ It is impossible that the condition will be fulfilled because it refers to the past.
Conditional Sentences Type III refer to situations in the past. An action could have happened in the past if a certain condition had been fulfilled. Things were different then, however. We just imagine, what would have happened if the situation had been fulfilled.
Form : if + Past Perfect , Conditional II (= would + have + Past Participle)
If I had found her address , I would have sent her an invitation.