Hypothesising about the unreal past
The third conditional expresses a hypothesis about the unreal past, that if, “if things had been different”.
It’s composed by a perfect subjunctive in the ‘condition’ and a past conditional in the ‘result’. That makes a minimum of five verbs, so learning the 3rd condition means making sure all the verbs are in the right form.
But only two of the verbs are main verbs, and the others are weak forms when they are positive, so the construction can be difficult to hear in fast speech.