Is money received from a governmental entity over three years, which was less than the number of deserved night shifts, and then turned into a salary without work, considered unlawful (haram) or rightful?

A private employee is entitled to wages as soon as the employer is enabled to benefit from their services, even if no specific task is requested. If your inactivity during that period was with the permission and knowledge of your employer, then there is no blame upon you for taking the salary, because the wage became due by your enabling of your service. However, if your inactivity was of your own accord, then it is not permissible for you to take the salary for the period in which you did not work, except with the permission of your employer. As for receiving a salary less than the prevailing rate, it does not matter as long as you have agreed to and accepted it.