Final answer:
The Third Estate formed the National Assembly to represent the common people and challenge the old class system, leading to political revolution and significant events such as the Tennis Court Oath and the storming of the Bastille during the French Revolution.
Step-by-step explanation:
The Third Estate utilized the Estates-General platform to initiate an act of defiance by establishing the National Assembly, marking the beginning of political revolution in France. This assembly represented the interests of the common people, challenging the outdated system which favored the clergy and the nobility through tax exemptions and political power.
After being locked out of their usual meeting place, the Third Estate members reconvened in an indoor tennis court on June 20, 1789. Here, they took the Tennis Court Oath, pledging not to disband until they had drafted a new constitution that reflected the principles of liberty, equality, and fraternity. King Louis XVI's attempt to dissolve this assembly with military force only fueled the revolution further, eventually leading to the storming of the Bastille and the rise of public uprisings across France.