The Third Estate was one of the three dividing bodies in France that had originated in the Middle Ages. It was renamed the National Assembly in June 17, 1989 as a result of having almost no say in the Estates-General that was called by Louis XVI. The Third Estate consisted of members of France who were neither part of the nobility, nor part of the church.
The overwhelming majority of the citizens of France were in the Third Estate, so had twice as many representatives (who were mainly educated professionals such as lawyers) in the Estates-General than the other two estates. This was of no importance, however, because all decisions made by the Estates-General was made by a vote in which each estate cast one single vote. The first two estates generally agreed with each other, so their two votes would always overrule the vote of the Third Estate. The system was purposely designed this way to limit the Third Estate's power.
The Third Estate requested increases in its power by suggesting such things as a vote by head, but as a result was banned from officially attending the Estates-General, and convened in a tennis court, which began its quest for more major reform of the French government (eventually leading to the Revolution).
Most information from lectures by professor Storch at the University of Wisconsin - Rock County. Additional information from Fourth Edition Western Civilization from 1300 by Jackson J. Spielvogel.