The Abbe Sieyes (Abbe is a title, not a name), believed in the importance of the Third Estate. He best laid his views on this out in a pamphlet that he published that was entitled "What is the Third Estate." In this pamphlet, he argued that the Third Estate was the most important part of France. In fact, he said that it was "everything."
The Third Estate, of course, made up the bulk of the French people. It was made up of everyone that was not clergy or nobility. Because of this, it made sense for Sieyes to say that it was "everything" even though it had very little power under the system of government that existed in France before the Revolution.
Sieyes ended his pamphlet with this statement that shows his view quite well:
The Third Estate embraces then all that which belongs to the nation; and all that which is not the Third Estate, cannot be regarded as being of the nation.