What is the ruling on the prayer of someone who intended to pray Maghrib, then it became clear to him that the Imam was praying Isha, and should he complete the Isha prayer with him or give the taslim after the third rak'ah?

The preponderant and fatwa-given opinion is the permissibility of following an Imam despite a difference in intention between the Imam and the follower. Thus, it is permissible for someone praying Maghrib to follow someone praying Isha. If the Imam stands for the fourth rak'ah, the follower praying Maghrib should sit for the tashahhud. He then has a choice: either to say the salam and then stand up, say the takbir for Isha, and catch one rak'ah with the Imam (and this is better), or to sit after his tashahhud without saying the salam, but instead wait for the Imam to say the salam with him.