Left index finger numb been this way for 3 weeks and wont go away

There. There are a wide variety of reasons for numbness, from carpal tunnel syndrome, to traumas causing neuropraxia. More information is needed for a proper suggestion on how to deal with your complain. Contact a hand surgeon in your area.
Try. Try splinting your wrist, especially at night. If it helps you may have some early problems with carpal tunnel syndrome.