Corys are pretty defenseless to a predator's attack, and they tend to be timid and less active when they are alone. But if you put in a good number of them, they feel more secure so they become more outgoing and active.
To me, 3 doesn't sound like enough at all. Personally I get at least 6 for a school. Cories are very sociable, and even though they usually don't all move around together at once, they still want to have more cories within visual range.
There are incredibly many species of cories and it's difficult to choose... Myself, I've got bronze cories, which seem to be among the least shy of the cories, but I also like panda and peppered cories.