How could the third estate lower high taxes?

The reason the Third Estate faced high taxes was that they lacked the legal privileges that the First and Second Estate had under the Bourbon monarchy. The clergy and the nobility were essentially exempt from most taxes. So in order to reduce the tax burden on themselves, the people of the Third Estate needed to enact taxes on the other two estates. A series of ministers under Louis XVI had attempted to make this happen, but these efforts were consistently voted down by the ultra-conservative Parlement of Paris, which was dominated by nobles. When the fiscal crisis of 1789 forced Louis to summon the Estates-General, a meeting of representatives from each of the three estates, the Third Estate delegates realized that their efforts to enact more equitable taxation would be defeated. This is why they formed the National Assembly and forced (aided by the constant threat of crowd violence) Louis to accept a series of reforms establishing a constitutional monarchy. Key to those reforms was the elimination of tax privileges that favored the nobility and the clergy. So as it turned out, the only way to establish more equitable taxation was through a fundamental alteration of the social and political order in France. This alteration marked the outbreak of the French Revolution, one of the most cataclysmic events in Western history.
https://www.history.com/topics/france/french-revolution