Shalom,
The message of the midrash which places Rivka's age at three in a Talmudic context seems to be about the amoral society of Haran. The midrash's assumption is that anyone over three in Haran would no longer be a virgin- if not for her own volition then through sexual harassment by the men of Haran. Rashi hints to this in his commentary (although he specifically focuses on the amoral behavior of the girls in Haran), and it also fits with the legal definitions of a virgin throughout the Talmud. I think this reading takes the focus off of Yitzchak and places it where Hazal wanted it to be- on the society of Haran.
However Yitchak's actions can also be better explained (if one wishes to take the Midrash's words literally) if we assume that Yitzchak and Rivka were only legally married for ten years until Rivka reached the age of thirteen and only then did Yitzhak consummate the marriage. Rashi himself brings this idea. While marrying a thirteen year old girl is not accepted in modern times, we can understand how it could be legitimate in other times and places.
Edited 1 time(s). Last edit at 11/06/2018 12:02PM by mlb.