You can deduct the medical expenses that you paid for in each year, regardless of when they got the braces. Ex: If you are still paying on the 2015 braces and made payments in 2017, then those 2017 payments are deductible as a medical expense if you itemize.
You can't deduct payments made in 2015/2016 in 2017. Those are deductible in those tax years. You can amend those tax years to take those deductions.
How to amend a previously filed return
**Say "Thanks" by clicking the thumb icon in a post
**Mark the post that answers your question by clicking on "Mark as Best Answer"