Where do I deduct third grade private school tuition

You may be able to deduct it as dependent care expense if you otherwise qualify. 

You cannot deduct private tuition as education expenses, unfortunately.  For federal tax purposes, education expenses (of any kind) for K-12 grades are not deductible. Some states do offer the deduction, be sure to go through your state interview questions. 

However, you may be able to deduct tuition as Dependent Care Credit if it was paid in order for you to work or go to school as a full-time student. Please visit:

1. Deductions & Credits
2. Child and Dependent Credit