Reciting tashahhud behind the Imaam

When your praying namaz behind the Imam, in every salah in the final rakat when you are seated in tashahhud, the minimum you have to recite is the tashahhud dua and if you start reciting durood and the Imam does salam then you have to do salam as well whether you have finished reciting the durood and dua or not, is this true? And in Zuhr, Asar, Maghrib and Esha, after the second rakat when you are seated in tashahhud and you are reciting the tashahhud dua, if you havn't finished reciting the dua and the Imam stands up for the 3rd rakat, do you finish reciting the dua and then stand up or do you follow the Imam and stand up even if you haven't finished reciting the tashahhud dua?