Conditional Type 3

The basic form of Conditional Type 3
main clause [would+have+past participle]
if clause [if+V in past perfect]
E.g: If we hadn't wasted time, we wouldn't have missed the train.
       I would have been more sympathetic is he hadn't accused me of lying.

Conditional Type 3 is used to refer to SPECULATE about PAST events, about how the things that happened or didn't happen might have affected other things.

It is used to express regret and reproach.
E.g: If you hadn't driven so fat, you wouldn't have had that accident. [reproach]
       I wouldn't have left my job if I had known how difficult it is to find another one. [regret]

It is also used to make excuses (but we can attribute this to the category of regret).
E.g: If there hadn't been an accident on the motorway, I would have been here on time.