There's an example:
He (kill) his victim easily, (remove) his wallet, (wipe) away all fingerprints and just (make) for the front door to make his escape when he (hear) a key turning in the lock. Someone (come) in! What he (do) if he (be) seen?
And I know that it should be: killed, removed, wiped, made, heard, was coming. But the answer in the last sentence is 'would do' and 'were seen'. I don't understand why I can't use 3rd conditional - 'would have done', 'had been seen'.
At the moment the man is thinking in 1st conditional "What will I do if I'm seen?" so, because the story is told in the past tense you have change that into the past tense and use the 2nd conditional.
You only use the 3rd conditional when talking about hypothetical options which you DIDN'T take.