From left to right, they are Rebecca, Miridia, and Catherine. These women serve as the Princess' personal maids. While they are charged with providing the Princess with all the physical labor she could possibly need, their primary task is providing the otherwise isolated girl with companionship. If not for these three, the Princess would have spent most of her life in solitude.
Miridia is the oldest of the three, and is the head maid, she serves as a motherly figure, not only to the Princess, but to the other maids as well.