Why did the third estate revolt?

To put it simply, the third estate revolted in response to an unfair economic and political system that disproportionately taxed the middle classes and peasants while benefiting the other estates. The first estate was comprised of higher-ranking members of the clergy and the second estate was the nobility. Everyone else, from poor farmers to wealthy merchants, made up the third estate.
In response to economic tension, Louis the XVI called for an assembly of the Estates General in 1789. Despite the fact that 98% of the population belonged to the third estate, all estates had an equal vote. The voting power of the clergy and the nobility together outweighed the rest of the population 2 to 1, so they used their votes to benefit themselves at the expense of the third estate. The third estate broke off into the National Assembly, ultimately leading to the storming of the Bastille and the beginning of the revolution.
Other cultural and historical factors contributed to the atmosphere of the French Revolution. Most notably, the rise of Enlightenment thinking in the eighteenth century challenged the supreme authority of the Church. The monarchy drew its power from the idea of “divine right,” where the king was supposedly destined to rule according to the will of God. When the Church was delegitimized as a moral and spiritual authority, the monarchy similarly lost its authority to govern. Additionally, the French Revolution occurred within decades of the American Revolution. All of Europe was watching as a colony of the greatest military power in the world gained its independence. It is interesting that one of the major issues at play in both revolutions was the idea of taxation without fair representation.
The French Revolution was a perfect storm of intellectual, social, political, and economic upheaval. All of these factors can be seen as intertwined, collectively leading to the revolt of the third estate.
https://www.mtholyoke.edu/courses/rschwart/hist151s03/french_rev_causes_consequences.htm