Until chapter 69, there has not been any mention yet of who Maria, Rose and Sina are in-universe. Neither has there been any explanation from the author as to who they supposedly represent. Though I have found this interesting interpretation as to what they might mean by kcwhitmore and gettingfitandstayingthere written on the snkconspiracytheories tumbler page. You can read it here or below.
First of all we have Wall Sina. The heartland of the civilization, the interior is an idyllic place, almost utopian in its bliss. This is accordance with the name ‘Sina’ which in fact means blessed. The reader/watcher was provided with a view into this blessed world when shown Pixis’ interactions with the government official. There was wine and food in abundance; beautiful rooms, buildings, and surroundings. This is in direct contrast to the outer most wall: Maria.
Even pre-attack, the interior of Wall Maria boasted none of the lavishness of the interior of Sina. This land was the outskirts of human domain with a scarce amount of food and other natural resources. The nature of this interior is reflected in the wall’s name; Maria means ‘bitter’. There is a decided bitterness to the life style within Wall Maria. Eren, Mikasa, and Armin did not grow up in a blessed land.
On top of that, Wall Maria was demolished by the Colossal Titan and the Armored Titan surged through the gape to break entry into the interior as a whole. This provided the titans with entry into the interior of Maria, forcing the humans back; decidedly dislodging their control in that land. This reflects the second definition of Maria, which is ‘rebellion’.
Finally we have Wall Rose. ‘Rose’ is a symbol for innocence, youth, and love. It is also the token symbol of many Great Goddesses, including Aphrodite. This love and youth and innocence is seen in Eren’s blissful ignorance as he declares the counterattack to have begun. For five years they lived in a tentative peace after the fall of Wall Maria. But that innocence was shattered by the appearance of the Colossal Titan once more.
The duality of a rose’s natures should also be considered when thinking of the Wall’s names. Rose’s though symbols of beauties are also dangerous because of their thorns. The walls as a whole function in this same regard. They protect the people, but they also keep them caged. There is a harshness to them despite the sanctuary they provide.
Overall the walls are, especially as considered by Wallists, three goddesses. These goddesses show truths of the world: there is bitterness (Maria) and blessedness (Sina)—a harsh duality (Rose) of the black and white society that are separated by the walls.