You will face these three sisters on the fifth floor of Tower of Zot, on the fourth floor take the right most door to get to the right section of the fifth floor. You have a save point in the first door you encounter, when you continue to the second door the battle will start.
This fight is easy the left girl cast reflect on the middle, the middle counter attacks and the right cast magic on the middle girl that get reflected to one of your party members, simply save the middle girl to last and take out the left first then the right. You might want to return and save after the battle. because there is one more boss waiting if you continue. and this one is much harder.