A brochure, a magazine or even an event invitation are all 3-dimensional forms that require the viewer to interact with the piece. They are a sculptural construction that require the same consideration of space and flow as one would expect from a 3D form.
I also think of painting as a 3D form, possibly 4D (if there is such as thing) when you take into consideration the dimensional effects color has on how you view the forms created.
Paint is a solid material that is moved about a canvas in much the same way clay is applied when making a sculpture. The dimensional application of the paint, or mark making, can really influence how one “reads” the painting… how the eye travels across the canvas. Think of the thick application of paint on a Van Gogh painting and the way the sculptural quality of the paint guides your eye around the canvas. Or the brush strokes on a Cezanne masterpiece.
Line, shape, color, texture are the tools 2-dimensional designers/artists work with to create the visual hierarchy, eye tracking, and an emotional connection that transforms a flat surface into a work of art.