Is it permissible for the Imam who made a mistake in the Tarawih prayer and stood up after two rak'ahs, to return to his prayer if the congregants alerted him, or should he complete the rak'ah and perform the prostration of forgetfulness (sujud as-sahw)?

If the Imam intends to make the taslim after two rak'ahs but then forgets and stands for a third in Tarawih, it is prescribed for the congregants to glorify Allah (say "Subhanallah") to him so that he may return. Then he should perform the prostrations of forgetfulness (sujud al-sahw).