What shall A Muslim do if he comes to the mosque to pray Maghrib in a congregation, but he finds them praying the first Rak’ah of Isha’ which has been combined with Maghrib?

Answer

Praise Be to Allah and Peace Be Upon His Messenger.
Referring to your question, according to the majority of Muslim scholars, combining prayers is allowed. Basically, the intention of the follower who prays with the Imam in a congregational prayer must meet the intention of the Imam. It is also recommended that the follower should come early to the mosque to get the full reward. Performing prayers at their appropriate time is one of the dearest deeds to Allah. 
Shaikh Muhammad Ahmad Hussain, the General Mufti (Muslim Legal Scholar), in the Palestinian Legal Verdict Council says: “Originally, Muslims can combine prayers due to rain, and this is allowed so as not to put them to hardship.”
However, Hanafi School Scholars say that combining each two prayers is not allowed but for pilgrims on the Day of Arafat on the Mount of Arafa and on Muzdalefa. during performing Hajj. 
Malki and Hanbali Scholars allow Muslims to combine Maghrib and Isha’ Prayers only. 
The Shafi’i Scholars allow Muslims to combine Dhuhr and Asr Prayers, and Maghrib and Isha’ Prayers because of rain under certain conditions which have been explained in their books. 
If a Muslim arrives to the mosque while the Imam is praying Isha’ which is being combined to Maghrib, whether the follower knows or not, he should have the intention of praying Maghrib, pray the first three Rak’at, stay sitting after the third Rak’a, say Tashahhud and the Abrahamic prayers and wait for the Imam to finish; once the Imam ends the prayer saying (peace be upon you), the followers can end their prayer (which in this case is the Maghrib prayer). Then, they can perform the Isha’ Prayer individually or in a congregation to get more rewards. 
If a Muslim arrives while the Imam has finished one Rak’ah, they can pray the rest three Rak’at with him having the intention of praying Maghrib. There is no harm in having different order regarding saying Tashahhud or Abrahamic prayers. 
If a Muslim arrives while the Imam has finished more than one Rak’ah, they can pray what is left to follow him, and then make up other Rak’at.
If a Muslim prays with the Imam the first three Rak’at, and then intends to end his prayer and stop following the Imam, he can do this; he can finish Maghrib Prayer, and then start again with the Imam to pray Isha’ so as to get the reward of a congregational prayers.