The feature that was added in Silverlight 3 is referred to as "perspective 3D" to avoid confusion that might arise with someone expecting it be some kind of full scale 3d rendering API (like DirectX or OpenGl etc.) or something like WPFs UIElement3D.
The "perspective 3D" feature in Silverlight allows you to take any regular 2D UIElement and rotate it in 3D dimensions; you can think of it like a more powerful RenderTransform. There are two ways of setting a "Projection" on a UIElement, with a PlaneProjection (simple) or with a Matrix3DProjection (more complex).
You could certainly use this feature to create a 3D cube, where each face of the cube is a UIElement with a Projection applied to it. However you would not use this feature to create something that required complex 3d models such as a 3D game.