What is incumbent upon one who completed his Hajj last year, and prayed the 'Isha prayer with the intention of Maghrib after its time had passed, then salam-ed in the third rak'ah, thinking it was the fourth?

If someone missed the Maghrib prayer and the 'Isha prayer has been called, they have three permissible options:

First: They can join the congregation with the intention of praying Maghrib. When the third unit (rak'ah) is completed, they should sit and wait until the Imam sits after the fourth unit. They then recite the tashahhud with him and make the taslim, and afterwards, they pray 'Isha. This is the preferred opinion of An-Nawawi and Ibn Baz.

Second: They can sit for the tashahhud after the third unit and make the taslim individually, separate from the Imam.

Third: They can pray 'Isha with the congregation, and then make up for Maghrib only, without repeating 'Isha.

All of these options are permissible, and the first one is the best.