Third Conditional

While the first conditional and second conditionals talk about the future. With the third conditional we talk about the past. We talk about a condition in the past that did not happen. That is why there is no possibility for this condition.
The third conditional is also like a dream, but with no possibility of the dream coming true.

Examples:

-If I had worked harder at school, I would have got better grades.

-If I had had time, I would have gone to see him. But I didn't have time


Notice that the main clause can contain 'would', 'could' or 'might.

-If I had seen him at the meeting, I would have asked him.
(But he wasn't there so I didn't.)

-If I had seen him at the meeting, I could have asked him.
(But he wasn't there so it wasn't possible.)

-If I had seen him at the meeting, I might have asked him.
(But I'm not sure. Perhaps if the opportunity had arisen.)

We can use the Third Conditional to talk about 'impossible' conditions, impossible because they are in the past and we cannot change what has happened. 

Examples: 

If she had studied, she would have passed the exam 

(but, really we know she didn't study and so she didn't pass) 

If I hadn't eaten so much, I wouldn't have felt sick 

(but I did eat a lot, and so I did feel sick). 

If we had taken a taxi, we wouldn't have missed the plane