Why did the third estate form the national assembly

In order to raise their voice against the injustice done to them by the feudal system, the third estate formed the national assembly.

Explanation:

The third estate was convinced beyond doubt that the other two legions made up of the clergies and the nobles would never take decisions in favor of the common people.They believed that for the good of the commons, the administration would be needed to be taken into their own hands.Hence, they formed the National Assembly in order to establish their might in the society dominated by clergies and nobles.