What is the ruling on the prayer of the majority who did not repeat the prayer after the Imam led them in the 'Asr prayer with three rak'ahs, then they doubted the number of rak'ahs?

The prayer of one who doubts having missed a rak'ah after finishing the prayer is valid. The congregation's prayer is also valid if they doubt having missed a rak'ah after their Imam has given the salām. However, if one is certain that they missed a rak'ah, they must perform it before a long time elapses. "A long time" is estimated by exiting the mosque or staying in it for a duration that is conventionally considered long. If a long time has elapsed, then the entire prayer must be repeated.