The names of the Walls always intrigued me and I thought of them being the founders for the said territories. Not just that they are mere wall names, but also they existed and were protecting the humankind from a greater threat. Yes, it could be that the three ladies be female titans (like Annie) and that they bound themselves to each wall.
But I think that more than 1 titan exist inside the walls. I mean, just look at that vast territory! And I don't think that its a coincidence that Annie happened to be climbing over that same wall. Since the walls are at intervals, sort of like a pillar which supports the ground, if you'll see, they are quite the number, I presumed that the each interval in the wall could mean per titan.
Or it could be that Isayama is laughing his head off while his fans are speculating crazily!