Scale Tool

Once you enable the 3D Option, the 3D Scale tool is available. You will see one of two things:

A Scaling Rectangle, this appears if your drawing is flat.
A Scaling Cube, this appears if your object has some rotated layers around the pivot points, indicating the different axes. If you click and drag on this Scaling Cube, it will scale proportionally on all axes.

Click on one of the three control points to scale the shape on one axis. Hold down the Shift key to scale the object proportionally. When the control point is selected, it turns pink.

If you move the layers in the Perspective view, it may be difficult to control the motion since you may not have the optimum point of view on your stage to understand the setup.