I actually did learn to read when I was three but:

1. I AM on the autism spectrum with comprehending hyperlexia (meaning I understand what I read, unlike some hyperlexics).

2. My mother was a teacher. She bought the flash cards and taught me.

So, I fit into BOTH of the categories you mention of people who really might learn to read that early.

Guess what I don't do?

Flex about it at parties. It doesn't matter forty plus years later. I know you probably won't believe me, but again it doesn't matter. I have written several books, I flex about THAT ;).