Have you spoken to support staff at your department? Or a medical professional?
You're not going to be able to get a decent grade just from using lecture material, especially in your third year. It's going to be extremely difficult if you have a dissertation or research project to write.
I don't have ADHD but I do have an awful attention span. Just set yourself really small goals, a couple of pages in a couple of hours. At least if you've read something you've got some material to refer back to.