The third conditional is only used about an established fact in the if- clause.
It doesn't have to be an established fact, but the speaker or writer must feel that it is a fact. If you're not sure whether it is the case, then we generally use the past tense, linking the unreal past with the unsure present. It has the idea of the present because the truth is not yet clear.
We can also use the second conditional for past facts\repeated actions, like a past zero conditional:
If it was wet, mum would make us stay indoors.
That was a lucky guess! ;-)