No, these things are not the same. Blisters are the result of wearing shoes that do not fit right, and working with tools, etc., without wearing gloves. The skin will form a blister like pocket. It is best not to burst the pocket, but let it heal naturally.
A corn is usually found on the feet. Often times surgery is required to remove a corn. At other times, for those that are less severe, a simple over the counter remedy may be used to remove the corn.
A callus is a rough spot that occurs, usually on the feet. Over the counter medications may cure this.