Not too young

I personally don't think 3 is too young for a 6 hr/week program. I think there will be an inital extinction burst when you attempt to go but it sounds like your kiddo would benefit with an additional couple of months of socialization before he goes to Kindergarten at 5.

If you are worried about the illness expect it to happen but ever since we've started the vitamin D/probiotics my DD and I have not really been sick for 2 years. She goes to all day preschool 5 days a week and I work in school districts. We just have really strong immune systems!