Taj mahal is must when you are in agra.
Day1 Taj mahal, agra fort and sunset at mehtab baugh.
Day2 in sikandara where you can visit tomb of akber, mariam tomb, chini ka rauza, baby taj.
Day 3 fattehpur sikri.
Please don't miss out on Agra Fort and Taj Mahal in early mornings; it looks beautiful and can give you chances of those unexplored shots .
Happy Traveling!!!