What is the ruling on praying three rak'ahs for 'Isha behind a congregation praying Maghrib, knowing that I was traveling and combined Maghrib and 'Isha?

According to the Shafi'is, it is permissible to pray 'Isha behind someone who is praying Maghrib, because they do not stipulate that the intention of the Imam and the follower must be identical. However, it is not permissible to shorten it; rather, it must be completed as four rak'ahs, because if a traveler follows someone who is completing the prayer or praying a prayer that cannot be shortened, then he must complete it. Therefore, the 'Isha prayer must be re-prayed as four rak'ahs if it was prayed incompletely.