The 3D Skybox is a 3D cube, and the camera and all other geometry is positioned inside the cube, the skybox serves as a permanent, static background on all 6 planes.
Therefore 6 images are required, top, bottom, left, right, front and back, to also give the false impression of depth for various scenes.
This scene was rendered using this skybox and was done as part of the course offering at Gameinstitute:
This is the gallery from this project. The skybox images were created from 3D geometry, rendered and captured to image at 6 different angles, that represent the inside of a cube.