Is it obligatory for me to make the Taslim with the congregation after the third Rak'ah if they are praying Maghrib (travel prayer) and I am praying 'Isha, knowing that I have completed my prayer and they have begun the 'Isha prayer (travel prayer), and is it permissible for me to be prostrating while they are standing, or vice versa, while I am standing in the middle of the row?

If you prayed Maghrib while traveling and joined them for 'Isha prayer with the intention of combining it with Maghrib, then your prayer is valid, even if they were praying Maghrib. You have the choice between completing the prayer with the Imam until the tashahhud and then completing your prayer after he gives the taslim, or separating from him after he sits for the tashahhud and completing your prayer. However, if you prayed Maghrib a long time ago, or if you did not intend to combine 'Isha with Maghrib, then your prayer is invalid and you must repeat it. And if you are not traveling and you prayed 'Isha before its time, then your prayer is invalid and you must repeat it, based on the Almighty's saying: "Indeed, prayer has been decreed upon the believers a timed ordinance." But if this occurred during the time of 'Isha after the first congregational prayer, then your prayer is valid if you intended 'Isha and prayed it four rak'ahs.