Yea, cory cats definitely should not be kept by themselves. Having a group of three should be fine though. I only have two, and although they would probably be happier if I could have more, I realistically can't, and they do just fine. I think the misconception that is most harmful to cories is that they can be kept one to a tank, for the sole purpose of acting as a clean up crew. They need company of their own kind to be happy, as well as to be fed w/ sinking pellets. I feed my cories sinking pellets only about twice a week, because they make the tank dirty. They do find food my gourami doesn't eat, but he is such a pig that they really need to be given their own food.