g73, it seems to me that Rashi is saying that Yitzchok waited until she was 3 to marry her because she was then “raoi l’biah”; this implies that they did indeed “get married” when she was that age. At age 13, according to Rashi, Rivka was capable of becoming pregnant, so that Yitzchok waited 10 years from that point before concluding that she was an akarah.
