I am looking for the source where I recall it written that Yitzchak had to marry Rivka at the age of 3 because he had to marry a virgin like the Cohen Gadol (High Priest). I think it was in the Talmud or Midrash Rabbah or somewhere. The Torah says he married her because she was a virgin. Can you help me? Thank you!

The Meshech Chochma (Breishit 24, 67) writes that Yitzchak's level of holiness was like a Kohen Gadol, and accordingly was not allowed to leave the Land of Israel. This doesn't necessarily mean that Rivka was 3 years old, for there are various opinions on the issue how old she was (as the Meshech Chochma himself cites), and the Torah says explicitly that she was a virgin, even if she was older.