If a person who is travelling wants to combine magrib and isha. However prays in the congregation while they are praying 4 rakats would it be correct for him to remain seated in the 3rd rakat

Let him finish his salah and then stand up and say "Allahu Akbar," joining them in the last raka'at as well. He prays four raka'at and completes the three rak'as that are remaining for him, even if he's a traveler.