There is a freeware package available called "Cereal Boxer" that does exactly what you want. It uses 3 bitmap images you create (for the front, top and side of the box) then renders a 3D image using these bitmaps. You also have the ability to control the "camera" position, angle, and tilt, as well as the lighting source and intensity, and it can create a "cereal box" of any size. It's a self-contained program so there's no installation involved.
Google on the phrase "Cereal Boxer" and you will find numerous locations to download the application.