Im superstitious and knock on wood often. sometimes i knock 3 times on my forehead like when you knock on door. could this cause concussion?

No: That type of knocking onto oneself does not cause concussions. Aside from events like motor vehicle accidents and major impacts in contact sports, typical people might encounter a head impact strong enough to cause a concussion when they fall down backwards and hit their heads on the floor. Lesser impacts than that are generally safe unless symptoms show otherwise.