Should a traveler pray Maghrib between the Adhan and Iqamah of Isha, or should he wait until the Imam prays Isha and then pray Maghrib with him, sitting during the third Rak'ah while waiting for the Imam, knowing that he is a frequent traveler?

If you arrive in your country before the entry of the 'Isha' prayer time, you must pray Maghrib immediately before its time expires. It is not permissible for you to combine it with 'Isha' because the ruling of travel ceases upon your arrival.

However, if the 'Isha' prayer time has entered before your arrival, it is permissible for you to delay the Maghrib prayer to combine it with 'Isha'. You would pray Maghrib first, then 'Isha' with the congregation.

If your prayer coincides with the 'Isha' iqamah, you can join with the intention of Maghrib. You would sit after the third rak'ah, waiting for the Imam to say the taslim, or you can say the taslim before him after the tashahhud and sending blessings upon the Prophet (peace be upon him), then pray 'Isha'.

The arrival that severs the ruling of travel is the traveler's arrival at their place of residence, or the residence of their wife with whom they have consummated marriage, or their staying in a place for more than four days.