O third conditional imagina um passado diferente do que aconteceu. Não dá mais para mudar: só imaginar.

if + had + particípio, would have + particípio

• If I had known, I would have come earlier. — Se eu tivesse sabido, teria vindo antes.
• If we had taken the train, we would have arrived on time. — Se a gente tivesse pegado o trem, teria chegado na hora.

Had + particípio é o past perfect (tivesse ou tinha + particípio). Nos verbos regulares, o particípio termina em -ed, como o passado: missed, invited, stayed. Os irregulares mais úteis:

Infinitivo	Passado	Particípio
go	went	gone
know	knew	known
meet	met	met
take	took	taken
see	saw	seen
come	came	come
be	was / were	been
get	got	got (nos EUA, gotten)