Rashi famously says Rivka was indeed 3 (25:20). He bases this on calculations that Yitzchak was 37 at the Akeida, that Sara died at that point & that Rivka was born then. Yet none of this is explicitly stated in the text; these are Midrashim that would have to be taken literally. But Midrashim aren’t always meant to be taken literally; Rav Avraham b. HaRambam says that while some are indeed literal, others are metaphors intended to teach us moral lessons (indeed, numerous Midrashim actually contradict one another). Other opinions (e.g. Ibn Ezra) say Yitzchak was 13 (or 26) at the Akeida & that Rivka was 13 (Tosfot). Still others (e.g. the Sifri) say that Yitzchak married her at 3 but did not consummate the marriage until Rivka was 14 years old (it was common for Jews in Arab societies to betroth young girls to save them from being taken by non-Jews, & then only marry them much later). The Rebbe wrote that Rivka was physically 14 but spiritually she was as pure as a 3-year old.