If each estate got one vote in the old system, why was the Third Estate weak despite having 600 representatives?

Because the number of representatives did not directly turn into votes

The governing concept is voting by order, or voting by estate, rather than voting by individual representative. The First, Second, and Third Estates each traditionally had one collective vote. Thus the Third Estate’s approximately 600 deputies did not automatically give it 600 votes. If the clergy and nobility voted together, the two privileged orders could defeat the Third Estate by two votes to one. The Third Estate therefore demanded voting by head, in which every deputy would count separately. Option A is correct because it identifies the mismatch between numerical representation and actual decision-making power. Option B is false because the Third Estate included most common people; C and D contradict the historical situation.

This is a Class 9 History question. Chapter: The French Revolution. Topic: French Society During the Late Eighteenth Century.