مغرب کی تیسری رکعت میں شامل ہونے کا طریقہ
It is a matter of Ijtihad. After the Imam finishes, you can either start your prayer by counting the first rakat as first or you can count it second with respect to the rakat of Jamat you could get. In the first case, you will not do Qada and if you consider it the second rakat then you will do Qada.