See Rav Shimon Shwab “MaAyn Bais HaShoava”. He tries to reconsile the Rashi (aged 3) and Tosfos by (aged 14) by saying that Rivka was born 14 years before she got married. Yitzchok was only shayich to become one of the Avos because of his willingness to partake in the Akeida and be the Korbon, which was three years before he married Rivka. Also, Sara Imeinu was nifteres at the time of the akeida. At the point at which Yitzchok was shayich to become one of the Avos, Rivka began her path to gadlus to become one of the Imahos. (Sort of Hikir es bora)
She was 14 years OLD, but 3 years IN THE MAKING.
Look it up. He is obviously much clearer than what I can say from vague memory.