If the question you're asking means only three types of food every day for all of your life...or for many, many months, it would have to be three foods which would keep you healthy during that time. You might not like them much but here would be my choices based on that criterion.
Brown or unshelled rice (it is supposedly the most balanced cerial but the rice husks need to remain on) an ocean fish such as Snapper or Tuna, and the third would need to be a green leaf vegestable - probably cabbage. I think with these three you'd be able to maintain the nutriments necessary to maintain health.
These are only guesses, of course, I am not a dietrician or food expert of any kind. This is just what I've gleaned from my reading over the years.