I guess I am advocating for safety for the snakes. For hobby breeders, what's the downside in waiting until 3 years as well as 3 feet & 300 gms? I'd rather skip that one clutch to reduce the risk to the snake, especially if female. For the boys, if they are willing, the only risk I can think of is that he may not be fertile even though he's willing.
Mind you, if Kathy Love, or Don Soderberg, or Rich Zuchowski, or Jeff Mohr, or Chuck Pritzel, or lots of other really knowledgeable people, looks at a snake & thinks "ready to breed" they know profoundly more than me, so I wouldn't advocate for them to wait for that rule of 3s. They KNOW what they are doing. Heck, their left pinkie fingers know more about cornsnake breeding than I do.
I just think the rule of 3s is really good for people who are new to breeding cornsnakes. It keeps our snakes safe while we gain experience.