The third conditional is about the past. However, it is about something in the past that did not happen. For example: if I would have seen Melissa yesterday, then I would have told her The truth. The 3rd conditional is speaking about a scenario in the past that is impossible. You didn't see Melissa, so you couldn't have told her. Both things are impossible. Another example:
If I would have won the lottery, then I might have travelled the world.
It's like a dream that was never going to come true. Hope that makes sense. You didn't win the lottery so you couldn't have travelled the world. Both refer to the past, but they were things that never really happened in the past. More examples:
I would have told Mary if I had seen her.
I would have invited Tara if she had been free yesterday.
Their teacher would have been sad if they had not passed their exam.
Would you have stayed at home if it had rained yesterday?
What would you have done if it had rained yesterday?
If I had seen Mary I would have told her.
If Tara had been free yesterday I would have invited her.
If they had not passed their exam their teacher would have been sad.
If it had rained yesterday would you have stayed at home?
If it had rained yesterday what would you have done?