If I join a group Salah during the third rakah, how I should perform the missed rakat? After Surah al Fatihah, I have to recite an additional Surah; because in farz Salah, it is only in the first two rakat that we have to recite an additional Surah after Surah al Fatihah.
You should complete the salat with the Imam, and if you missed some Rak'aat, then after the Imam say the Tasleem, stand up and make up the Rak'aat that you have missed. The Prophet (peace and blessings be upon him) said:
إذا أقيمت الصلاة، فلا تأتوها وأنتم تسعون، وأتوها وأنتم تمشون، وعليكم السكينة، فما أدركتم فصلوا، وما فاتكم فأتموا
When the Iqamah are pronounced, do not come to it running, you should walk calmly with tranquillity to join the congregation. Then join in what you catch for and complete what you miss