A 3D Camera is a tool used in game engines to display elements in three dimensions and convey depth to the viewer.
This is typically done using a coordinate system with X, Y, and Z axes.
2D (two dimensional) images are commonly used with a 3D camera in visual novels, but may also be done using 3D images in certain game engines.
Use in Ren'py
Ren'py uses a 3D Stage to position its 3D Camera and both 2D and 3D displayables.