While Dr. Hendel's explanation is interesting, it is somewhat forced, particularly if one looks up the midrash from which Rashi gets the idea.
The midrash is clearly suggesting that Rivka was three, and, as I had conjectured before seeing the midrash, that Rivka was "pure" when she married Yitzchak (Ayein Sham).
Why is Rashi quoting this midrash? I have no idea. I would keep in mind an idea told to me by Rabbi Howard Bald, the principal at Hillel in Deal NJ, when he was my boss. One should not teach Rashis and midrashim that they do not understand and/or can not explain. Lacking a less forced explanation than Dr. Hendels, I would simply skip that Rashi.
