Question Summary:
While reading salat (namaj) with jamat, in magrib salat, in third raqaat, when the imam is reading the “alhamdo and then another surah” in his mind (not loudly), what we have to do?

Question Detail:

While reading salat (namaj) with jamat, in magrib salat, in third raqaat, when the imam is reading the "alhamdo and then another surah" in his mind (not loudly), what we have to do?
Read the alhamdo in mind or not?

Answer :

In the name of Allah, Most Gracious, Most Merciful

Assalaamu `alaykum waRahmatullahi Wabarakatuh
 
In the third rak’ah of Maghrib, as well as the third and fourth rak’ahs of Zuhr, ‘Asr, and ‘Ishā’ prayers, the imām will only read sūrah Fātihah and he will not read another sūrah after it.  As for those following the imām, they will remain silent and not recite anything from the Qur’ān in all of their rak’ahs in each of the prayers.  They will only read the thanā’ in the first rak’ah. 
And Allah knows best
Wassalaamu `alaykum
Ml. Abrar Mirza,
Student Darul Iftaa
Darul Iftaa, Madrassah In'aamiyyah