ANSWER:

You should recite the salutations upon Prophet salla Allahu alaihi wa sallam and can even say the dua until the imam gets up for the third rakah, instead of sitting idle.

Sheikh, if I finish Tashahud before the Imam while sitting after the second Rakah before getting up for the third rakah, should I keep on reciting Darud until the Imam gets up for third or should I keep silent and wait?