ANSWER:

You should join the congregation with the intention of maghrib and after the third rakah, sit down and finish your maghrib quickly and rejoin the Imam with the intention of isha while he’s still praying his isha and after he concludes, you get up and continue your isha. 

 

I missed magrib prayer due to a valid reason and when entered the masjid, salatul i’sha was announced. Should I join the same or need to make up the magrib first?