Three daughters
My theorie about why it took three tries to get a "good" daughter:
The robots try to raise daughter one, but they don't know how, messed up and the daughter has to be aborted.
The robots decide to place the second daughter as a baby near the people who are still there to see how to properly raise a child. She becomes Woman.
With the learnings they try a third time and raise Daughter properly.
More idea's:
Woman is used as a test to see wheter Daughter will return to the facility. When Daughter says she is special, the robots know she is ready for the task to repopulate the earth. (Similar to the Matrix theme). The bad robots stop invading the facility and Daughter kills Mother and Woman is no longer required. At the end the eyes of Daughter look dark, suggesting she now replaced Mother.