assalamu alaykum. supppse i join imam in the 3rd rakah of zuhr salah. after salam i stand to complete my missed portion. then i noticed that imams wudu had broken in salah or before salah, so they start to repeat the salah. what should i do then?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The most authentic opinion is that in case it became clear - after finishing the observance of prayer - that the Imam has already broken his ablution, the prayer of those led by him in this prayer is still valid.

They are not obliged to repeat it. As such, the prayer of those people led by this Imam and who realized – after the observance of prayer – that his ablution had been broken is valid, and they are not obliged to repeat it according to the most authentic opinion on that issue. On that, Sheikh Ibn ʿUthaymeen was quoted, “This is a useful ruling that I would impart to you right now; all praise is due to Allah that this question is offered now. If the Imam observes prayer while his ablution is broken and he does not know about that except after the observance of prayer, he has to make an ablution and repeat the prayer. As for those led by him in prayer, they are not required to do anything at all.

However, if he comes to know during the prayer that his ablution has been broken, he has to leave the prayer and say to one of those led by him, 'O so and so! Complete the observance of the prayer as an Imam'. In case one Rakaʿah is remaining, they should observe one Rakaʿah only under the leadership of the new Imam. But, if he does not say anything, those led by him should ask one of them to lead them. Yet, in case they won’t ask any of them to lead the prayer, they should observe it individually.” Under the case you mentioned, you are not required to do anything but complete your prayer.

Allah knows best.