What does the mri mean when it mentions diffuse central canal narrowing from c3 through c6 levels which is likely congenital?

You've always had it: "Congenital" means since birth. Diffuse central canal narrowing means that your spinal cord in the neck region doesn't have a lot of wiggle room. Unless you are having neurological symptoms or the report mentioned a critical stenosis or change in the cord signal, you should use the MRI as a reference scan. Otherwise see a neurologist. Ref: http://emedicine.medscape.com/article/344171-overview#a21