Is it obligatory for the congregants to repeat the Asr prayer, or only for the Imam, if the Imam prayed three rak'ahs with the intention of Maghrib prayer, forgetting, then completed the fourth rak'ah and performed the prostration of forgetfulness, while the congregants' intentions were for the Asr prayer?

If the Imam prays Asr with the intention of Maghrib, his prayer is not valid due to the absence of a correct intention. And if he changes his intention to Asr during the prayer, it is also not valid. As for the prayer of the congregants, it is not invalidated by the invalidation of the Imam's prayer in this situation, because their prayer is valid as long as they are excused and unaware of the invalidity of their Imam's prayer. The error falls upon the Imam alone, and the congregants are not required to repeat their prayer.