Conditional sentences of the third type are used to describe conditions in the past. If something had been the case, the present state of things would be different now. The third type always refers to the past and can never become real.
If-clause in the past perfect + main clause with "would/could/might" + "have" + past participle
Take a look at the following examples to get an impression of how this sentence type functions!
If I had not left so early, I would have been able to see it myself. (But I left early and I did not see it myself.)
If it had not rained all day, we would have gone on a bike tour. (But it rained all day and we did not go on a bike tour.)
If Martha had told me that before, I could have been there at the right time. (But she did not tell me, and I was too late.)
Now try that out yourself. In the exercise below, you always get two sentences and you have to combine them into one, using a conditional sentence type three. Good luck!