Mistakenly standing up after the second rakaat without sitting in qadah

If instead of sitting for tashahhud in the second rakaat, you had stood up for the third rakaat, then if you are performing a four rakaat fard salaah, three rakaat witr salaah or four rakaat sunnat-e-muakkadah of zuhr or jumuah, then in all these cases you should not return to the sitting posture, rather you should continue in your salaah and make sajdah sahw at the end of the salaah.

However, if you are performing four rakaat sunnat-e-ghair muakkadah of asr, four rakaat nafl or two rakaat sunnat-e-muakkadah, then in all these cases you should return to the sitting posture and make sajdah-e-sahw at the end of the salaah.

And Allah Ta'ala (الله تعالى) knows best.