Who were unfairly taxed in France for hundreds of years? (around french revolution era)

Those unfairly taxed in France were the peasants and working class also known as the Third Estate. Under the King there were 3 Estates, the First being the Clergy, Second being the Nobility, and the Third being everyone else.

Some taxes included Corvee, a working labor tax which was first introduced in 1737. Peasants were forced to work on the noble's land such as roads while being unpaid, and the Gabelle, an unpopular tax on salt that was given to the third estate.