Ive seen this mentioned 3 times now on the internet that The Rebbe wrote that Rivka was physically 14 but spiritually she was as pure as a 3-year old. I read his sichos though on parshas and he says she is literally 3. Would love to find a source for this alternative opinion since allot of people teach/ take rivka being 3 literally.

ב"ה Shalom, Firstly, allow me to say that I cannot comment on that which was said by the Rebbe, without having seeing it in his words. But the discussion regarding Rivka's age at the time of her marriage to Yitzchak, began long before the Rebbe. There are three major opinions about the age of Rivka. The most well know opinion is that Rivka was 3 years old. This is quoted in Rashi (Bereshit 25:20) . The source of this is in the Midrash Seder Olam Rabba Chapter 1. The premise of this calculation is the juxtaposition of the torah telling us of Sara's death at the age of 127 as a result of the Akeda. Since Sara was 90 when she bore Yitchak, the result then makes Yitzchak 37 at the time of the Akeda. Since the birth of Rivka is mentioned at that time, and the fact that Yitzchak was 40 when he married Rivka, this makes Rivka 3 years old at the time of her marriage. [However, it should be mentioned that the Vilna Gaon in his emendations to the Seder Olam corrects the version of Seder Olam and writes that Yitzchak was 26 at the time of the Akeda, and Rivka was 14 when they married. (See also Torah Shleima to Bereshit 22 note 108)] A second opinion is that Rivka was 14 years old. This is based on the Sifre Devarim 357 and the Midrash Hagadol to Bereshit 25:20. The premise for this calculation is that Rivka lived to the age of 133 as did Kehat. The deduction of different periods in her life from the time of her death place her at the age of 14 when she married Yitzchak. This opinion is also cited by Tosafot Yevamor 61b in the name of Rav Shmuel Hachassid of Speyer. The third opinion is the Talmud itself (Yevamot 61b) which quotes the Torah where Rivka is called a "Naara" which then places her between age 12 and twelve and a half. Tosafot (ibid) mentions all three opinions reconciling them by simply saying that the Midrashim are conflicting. As aforementioned, Rashi on the Torah says that Rivka was 3 at the time she married Yitzchak. Although, getting married at the age of three is totally foreign to the world we live in now, and Rivka being only 3 poses other questions , the concept of marriage at the age of three should not come as a total surprise since it is mentioned in the Mishna (Niddah 5:4) that when a woman is 3 years old she can be betrothed with the consent of her father and this is codified in the Rambam and Shulchan Aruch, though it is not in practice nowadays. (See also Rabbenu Bechaye to Bereshit 24:19 and Zohar to Toldot 17) What concerns some of the Rishonim (commentators ) Ramban, Rashba, Meiri , Tosafot Rosh in their commentary on the Talmud (Yevamot 61b) is not the fact that Rivka was 3 years old, but the fact that the Torah calls her a "Na'ara" which denotes an older age. They entertain the idea of the version of Rivka being 14 at the time , but are unconvinced about it. To sum up, though I have not seen what the Rebbe said on this issue, the earliest sources on the subject of Rivka's age at the time of her marriage are in conflict which leaves room for commentary and requires a deeper understanding. All the best. Shanna tovah.