There is no basis in the Sunnah for saying du’a between one salam and the other during Tarawih prayer by saying “Subhanaka Rabbana wa bi hamdika Allahumma ighfir lana.”

There is nothing wrong with repeating the same Surah in the Tarawih prayer, or in any other prayer. A person may recite a Surah in the first Rak`ah and repeat the same Surah in the second.

Du’a al-Qunut is recited in the last rak’ah of Witr prayer, after bowing. The formula of Du’a al-qunut was narrated by Abu Dawud, al-Tirmidhi, and al-Nasai.

It is permissible to pray Witr 3 rak’ahs with one salam, but it should be with only one tashahhud and not two, because if he does it with two tashahhuds, it will be like Maghrib prayer and the Prophet (peace and blessings be upon him) forbade making witr like Maghrib prayer.