Floaters are a common problem and even i am having it. When you have floaters and flashes only thing one should do is to get check retina for any weak areas that can later become a retinal tear. U have already done this 3 eye doctors. So don t be panic about it. As far as seeing shadows of bright things like computer screen is a normal phenomenon which is called after image. You are getting aware of it as you are getting over-conscious about your vision. So be calm try relaxation exercises to avoid any stress.