Some thoughts:
1. I think Kumon is not the ticket. It seems to be based on repetition to achieve mastery, which some gifted children at least seem to find boring. Montessori or some other preschool might be a much better option.
2. You could after(pre)school him. There's no need to pay for a tutor, unless your time is so valuable that you can't spare any. It could be a time to share with your child and have fun. This would be far superior to Kumon, in my opinion.
3. The handwriting practice is valuable. Lack of handwriting skills seems to often be an obstacle to early grade acceleration.
4. That said, a poor fit is a poor fit. You should consider that for highly advanced young ones, you might not be able to find a good preschool fit. It happened to us; we took DS-now-6 to a highly touted farm school in the area, where they were supposed to encourage academic development, and he was bored out of his skull.
5. You generally can't demand anything from a preschool.
6. You can also challenge your three-year-old without moving toward rigorous academics. There are plenty of play-based and building activities that could provide challenge. Monopoly can build on simple calculation skills; ThinkFun and other games such as Logic Links can foster logical development; building sets such as Capsela, Erector, Lego, etc. can develop visual-spatial problem solving; studying art history with parallel art projects can encourage art ability and appreciation; etc. Even jumping in mud puddles can be a learning experience at that age; keep your options open and respond to his interests. There's plenty of time for academic focus and acceleration later.
