If a worshipper forgetfully stands up for the third unit of a Fard prayer, omitting the first sitting, and they have already stood up straight, what is the correct procedure?

They must immediately sit back down and continue.
They must not return to the sitting, complete the prayer, and perform Sujud al-Sahw at the end.
The prayer is invalidated and must be restarted.
They must perform a prostration of recitation.