What is the formula nitrogen trioxide?

There are two compounds that you may be thinking of. The stable compound known as Nitrogen Trioxide is also known as Dinitrogen Trioxide, which has the molecular formular N2O3. However, there is an intermediate compound formed in some multistep reactions with the molecular formular NO3, which you could say suits the name better. This compound is unstable, however, and cannot exist for very long without reacting.