Nuke’s Card3D node lets you apply the same geometric transformations possible with the Transform node, but gives you an additional axis of operation, z.
Just to be clear, the Card3D node’s transformations are not truly 3D, but rather what is sometimes called “2.5D” - meaning that you can move an element back on the z axis, but doing so does not convey the sense that it is behind or in front of another element. 2.5D transformations are useful for tasks like “cheating” the perspective of an element or “faking” a camera zoom.
Remember, however, that Nuke doesn’t limit you to 2.5 dimensions. If you need true 3D capabilities, you can construct a 3D scene. See 3D Compositing for more information.