When you say "one of a different kind," do you mean another cory? What was the emergency that made you take the one fish out?
Cories like to travel in little packs - they can be very shy if they don't have other cories to hang with. If all three fish are cories, then yes, having a lone cory in a tank is not good for the fish.
Sometimes I get the feeling that the whole world is against me, but deep down I know that's not true. Some of the smaller countries are neutral. Robert Orben