what are examples of 3-d objects
Sculptures.
http://en.wikipedia.org/wiki/Dimension
When at the link I left you above notice the objects on the right. A dot, line, rectangle, cube. The cube represents a 3 dimensional object. Anything in our "real" world is percieved through sight (and touch) as having 3 dimensions. Length width height. That means cars, dogs, people, houses, etc are all percieved as 3D
Mobiles, origami and pottery are all 3D art.