When working in 3D, the drawing or layer pivot becomes a location that can move on three axes. When you work in two dimensions, the pivot is mainly an anchor point somewhere in the drawing from where the transformations will be performed. In 3D, the transformations are made on three axes, so the centre of rotation and scaling also becomes three-dimensional.
By default, the pivot is positioned at the centre of the stage: (0,0,0). When you use the Pivot tool to reposition the centre of transformation, the pivot position is changed on the X and Y axes, but the Z-axis remains at zero until you change it in the Top, Side or Perspective view using the transformation tools.