The Imam offering the third Rak`ah missed of the Maghrib Prayer and performing Sujud-ul-Sahw in case of saying Taslim after the first two ones

Q 2: One day we offered the Maghrib (Sunset) Prayer behind an Imam (the one who leads congregational Prayer) who concluded the Salah with the Taslim (salutation of peace ending the Prayer) after only two Rak‘ahs (units of Prayer). He then wanted to repeat the whole Salah (Prayer). I told him it was enough to offer one more Rak‘ah, but he said that they wanted to repeat it. Which is the best, to repeat the Salah or to offer the one omitted Rak‘ah? (Part No. 6; Page No. 26

A: Anyone who unintentionally omits a Rak‘ah in the Maghrib Prayer should get up and offer a Rak‘ah, recite the final Tashahhud (a recitation in the sitting position in the second/last unit of Prayer), say the Taslim, offer the two prostrations of Sujud-ul-Sahw (Prostration of Forgetfulness), and then say the Taslim again. They do not have to repeat the whole Salah; because they have not intentionally omitted the Rak‘ah. When the Prophet (peace be upon him) mistakenly omitted one of the actions of Salah and said the Taslim, then his attention was drawn to it, he made up for what he omitted, said the Taslim, then performed Sujud-ul-Sahw. He did not repeat the whole Salah. Allah (Glorified and Exalted be He) says: Indeed in the Messenger of Allâh (Muhammad صلى الله عليه وسلم) you have a good example to follow The Prophet (peace be upon him) said: “Pray as you have seen me praying.” (Related by Al-Bukhari in his Sahih [Authentic Hadith Book]) However, if the intervening period before the Rak‘ah is made up for is longer than is customarily acceptable, the Imam and the Ma’mums (persons being led by an Imam in Prayer) should repeat the whole Salah.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.