Once you enable the 3D option, the 3D Scale tool is available. You will see one of two things:
|•||A scaling rectangle appears if your drawing is flat.|
|•||A scaling cube appears if your object has some rotated layers around the pivot points, indicating the different 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. Once the control point is selected, it will turn pink.