Like a lot of Tanach Tales, the idea that Rivkah was a mere three years of age when she married 40-year old Yitzchak has a very fine pedigree. Rashi cites it, and so do many others Great Names. (But it isn't in the Torah.) If you'd like to imagine that Yitzchak took a wife who needed a two-hour nap, and help getting dressed, go ahead, you have plenty of support. Rashi says it outright in his comment on Genesis 25:20, and seals the deal with some math.*
But what about the rest of us? How do we skeptics address the idea that one of the greatest Jews married a toddler?
The problem here isn't with Rashi or the midrshim he cites. The problem is that we got just one side of the story when we were in school. The teacher told us that Rivka was three, and that was it. We were too young to realize that this was seriously odd, and probably too timid to argue. Later, after our adult sensibilities kicked in, we had a choice to make: Should we believe this, or not? Muddying the waters are some elaborate justifications.
Luckily, there's a third way. We're Jews, remember? There are always other opinions, and in this case, the opposing views are every bit as authoritative as Rashi. Start with the Ibn Ezra. In his own comment to Genesis 22:4, he argues, rather convincgly that Yitzchak was not 37 years old at the Akeida. This matters, because the math Rashi uses to prove that Rivka was three, depends on Yitzhak being 37 at the Akeida. If Yitzchak wasn't 37 at the Akaida, Rivka was not three when she got married.
If this doesn't convince you (and it would if you weren't too lazy to check my sources) there's more: Read about it: here and here and here If you're feeling especially ambitious visit the Tosfot on Yevamot 61b (“v’chen”)
It'll make you feel so much better - so much more confidant and cheerful - the next time you roll your eyes at a True Beleiver who insists that Rivka was three and that's that.
*This is the math that Rashi uses to support his claim that Rivka was three when she married Yitzchak. As you'll see from the other materials I posted, Rashi's math is based on some assumptions that other challenge.
1. Sarah was 90 when Yitzchok was born.
2. Sarah died at 127 immediately after the Akeidah, so Yitzchok was 37 at the Akeidah.
3. Immediately after Sarah's death Avrohom hears the news of the birth of Rivkah, so she was born when Yitzchok was 37.
4. Yitzchok was 40 when he married Rivkah, so she must have been 3 when they married.