How does the third "if" conditional statement affect the present since it is used for events that cannot be changed in the past? E.g., "if you had known, you would have done the work last week."
If you had got a job last summer, instead of playing with your friends, you would have made some money, instead of having to ask me for some now.
I'm not sure that this is much different in English than in other languages, since there is generally little reason to bring up how a past event could have been different except to say how that difference would have changed the present:
Many postulate that if Adolf Hitler hadn't driven out all the top German Jewish physicists from Europe, Germany might have developed the atom bomb years earlier, and who knows what the world might be like today?
Since the protasis of the third conditional expresses a hypothetical past that, as you say, can never become actual, the effect of its non-occurence on the present is also irreversible. Although the effect indicated in your example is also in the past, philosophically it can be said to affect the present, which is the consequence of all previous causes in the universe, so that if that guy had known (say about the forthcoming test), and consequently had done the homework the previous week, that would have affected the present somehow, e.g. by relieveing him of doing that homework a week later instead of going to a birthday party.