The 3rd conditional is used to talk about something from the past that didn't happen. Because the action happened in the past, both the condition and the result are impossible now.

You can start with the 'If clause' and then the main sentence. In this case you will usually find a 'comma'(,) after the 'If clause'. The other way is to start with the main clause and then the 'If clause'. In this case you mustn't include the comma (,) after the first sentence because we know where the 'If clause' starts.

- If I had met (past participle) you last week, I would have told (would+have+participle) you I was going to leave the city.