Describe how enlightenment ideas influenced the members of the third estate and why they declared themselves a national assembly

Enlightenment ideas advocated the equal rights of all men, and called for more equitable and representative government. This led members of the Third Estate to declare that they should be the National Assembly to govern France.

Let's let Abbe Sieyes of France comment on this question for us.  (Abbe is French for "abbot" - head of a monastery.)  Emmanuel-Joseph Sieyes was a member of the French clergy (that was the "1st Estate"), but he was a huge advocate for the common people (the "3rd Estate.")   The Third Estate had some allies, like Sieyes, in the 1st Estate, as well as some within the nobility --  the "2nd estate."

In the days leading up to the French Revolution, Sieyes published a pamphlet that argued Enlightenment ideas, and had a huge impact in France.  The pamphlet was titled, "What is the Third Estate?"  Here's how he started that pamphlet:

The plan of this book is fairly simple. We must ask ourselves three questions.

The clergy (1st Estate) and nobility (2nd Estate) amounted to about 2% of the population of France in the days before the French Revolution -- but all political power was in their hands in everything.  The rest of the nation -- from lawyers and merchants down to peasants and street sweepers -- all counted as the 3rd Estate.  That body of the people made up 98% of the population. The time had come for them to take hold of their power in deciding how their country was going to operate.  When the Estates General met in 1789, the Third Estate ultimately declared themselves as the National Assembly.

b. second continental congress

the influence of charles louis de secondant , baton montesquieu.