Michael Schoch, Answers questions on Noodle
It looks like the Cobb School District has a special education preschool program that accepts 3 year-olds. I couldn't find out if the program was offered specifically at Varner Elementary School or not, so I'd recommend calling the school at (770) 222-3775.
You can also refine your search on the Noodle Preschool Locator to find schools near you that accept 3 year-olds.
Finding a good preschool is never the funnest thing in the world, but I hope the process goes smoothly for you. If you need help, feel free to ask a follow up question or check out some of the resources on Noodle!