After saying sending peace and blessings upon the Prophet (may the peace and blessings of Allah be upon him) in the middle tashahhud, you should stand up in order to start your third rak'ah (unit of prayer), and say the takbeer (Allahu Akbar) while raising your hands to the level of the ears or the shoulders. This is because of the following narration of Nāfi' (May Allah have mercy on him):
"Whenever Ibn 'Umar started the prayer, he would say: Allahu Akbar, and he would to raise his hands; and whenever he bowed, he used to raise his hands and also used to raise his hands upon saying, sami' Allahu li-man ḥamidah, and he used to do the same on rising from the second rak'ah. Ibn 'Umar said: 'The Prophet (may the peace and blessings of Allah be upon him) used to do the same.'" [al-Bukhārī: 739]
The third rak’ah (unit of prayer) should be performed in the same way as the first rak'ah, except that in the third and fourth rak'ah, only Sūrah al-Fātiḥah should be read, without adding any additional sūrah along with it, as it is proven from the narration of Abū Qatādah (may Allah be pleased with him):
"My father said that the Prophet (may the peace and blessings of Allah be upon him) used to recite al-Fātiḥah followed by another chapter in the first two rak'ah of the prayer, and used to recite only al-Fātiḥah in the last two rak'ah of the Dhuhr prayer. Sometimes a verse was audible, and he used to prolong the first rak'ah more than the second, and used to do the same in the 'Aṣr and Fajr prayers." [al-Bukhārī: 776]
After reciting Sūrah al-Fātiḥah, you should perform the bowing and then the prostration, as described in the first rak'ah. If you are performing a prayer which is only three rak'ah in length, for example the Maghrib prayer, then you should skip the rest of this section and go to the final tashahhud.
If you are performing a prayer which consists of four rak'ah such as Dhuhr, then after performing the second prostration in the third rak’ah, you should sit briefly and then stand to begin the fourth rak’ah, in the same way that you would stand up from the first rak’ah to begin the second.
In the last rak'ah of the prayer, whether the prayer is three or four units in length, you must sit for the final tashahhud.