Question

The illegal event that constituted the start of the french revolution was when the third estate declared itself

Answer 1

The National Assembly of France.

This was illegal according to the laws of the time, but the 3rd Estate was declaring those laws and rules as a system that opposed the will and well-being of the people of France.

The old laws on the books said that when a gathering of the Estates General occurred, each of the three estates met separately and each group had one collective vote (the consensus of the whole group) as their final vote.  The 1st Estate (clergy) and 2nd Estate (nobility), representing 2% of the population total, would combine their votes in opposition to the vote of the 3rd Estate (the common people).  The 3rd Estate essentially declared that they, 98% of the nation, were the nation and that their delegates thus were the National Assembly.

There's more to the story than that, but we'll keep it brief here for now!