If the First Estate was the church and the Second Estate was the nobility, the Third Estate was "everyone else." It comprised beggars, peasants, the bourgeoisie, notaries, doctors, lawyers, artisans, bankers and even scholars - who, at the time, represented approcimately 96% of the population.
* I don't think that means 96% of the population were scholars. If it did, this would be one terrifically nerdy time period. I dare say you and I would have fit in rather well.
In a pamphlet entitled What is the Third Estate? (published in 1789), Sieyès wrote: "What is the Third Estate? Everything. What has it been until now in the political order? Nothing. What does it ask? To become something. Nothing be done without it, everything would be infinitely better without the other two orders".