Can a concussion keep you in the hospital 3 weeks?

No: If actually in the hospital for three weeks, more problems than a concussion.

Depends on severity: Some people may have a post-concussive/traumatic headache. If the headache is accompanied by other symptoms (tinnitus, difficulty concentrating, irritability) then you might have post-concussion syndrome. Improvement in symptoms is based on many factors including severity and location of the concussion. See a neurologist for evaluation. http://www.webmd.com/brain/post-concussion-syndrome