The Third Conditional is used to talk about unreal situations, the things that did not happen in the past, and now we regret:
If I hadn't overslept, I wouldn't have been late for school.
(But I did, so I was late.)
If we had played better, we could have won the game.
(But we didn't, so we lost.)
Now, try some quizzes and tests here, here, here or here.
Or play the games here or here.
If clause
if + subject + past perfect,
main clause
subject + would / could / might +
have + past participle