The title speaks for itself, i can't understand the difference.
Let's take this example:
Type 2: If she fell, she would hurt herself.
Type 3: If she had fallen, she would have hurt herself.
For me both are correct and both are usable when you saw a girl jumping on the edge of a building in the past. Rules are saying that type 2 describes an unreal situation, but the girl jumping on the edge is real, and the possibility of her falling off is real too.