I personally am not a fan of Kumon for younger children. As Amanda pointed out, the playing field does seem to level out sooner than later.
The other reason I would encourage a family to wait is because of *how* the child's brain develops. When children are interested and asking questions about letters and numbers, we know their brains are ready to learn the information. There is a huge spectrum of time in which kids naturally manifest this interest. The child's brain is learning a great deal of things in its own time. Kindergarten steps in and introduces more information than a child would seek out on their own, and five is a perfect age to begin more of the direct instruction we see in public kindergarten.
Instead of Kumon and individualized instruction, consider a more peer-oriented environment for your daughter. A couple mornings of preschool a week would be a good place to start. For children your daughter's age, the social development piece is very important, it is a fundamental building block for kindergarten and elementary school. As a preschool teacher myself, I focus first on providing the support for children to master social skills like sharing and taking turns and being in the group; this helps to augment the more academic lessons and learning opportunities taking place throughout the day.
Mama's On Call had a great blog piece about the emerging trend toward academics at an ever-earlier age and the importance of free unstructured playtime in a child's life.
http://mamasoncall.com/2011/05/never-too-young-for-the-ra...
I will also say that from my experience with kids that when they are ready to learn information and showing interest, they do often pick things up more quickly than we would expect. I've also noticed that parents who stress academics at an earlier age seem to have more anxious kids. I think three years old is a bit too early to have to worry about making perfect letter and numbers for someone else's approval, or to have to do it again and again until it's correct. That would be a stressful job environment for an adult, too.:)