The third conditional describes an unreal situation with reference to the past.

Imagine a Hypothetical situation, somthing that did not happen:

 

 If + Past Perfect Tense would have + past participle

  If I had had enough money                ,    I would have gone to Japan.

Use:

  1. Nature: unreal
  2. Time: Past (so we are talking about a situation that was not so in the past.)
    e.g. If you had warned me, I would not have told your father about that party.(But you didn’t, and I have).

 

http://busyteacher.org/6063-how-to-teach-the-real-unreal-and-past-conditionals.html

http://www.englishgrammarsecrets.com/thirdconditional/menu.php