modifying X, Y and Z scale values as a group in keyframe editor

Use the transform keyframe tool in the keyframe editor. It's the far right tool at the top of the lefthand part of the keyframe editor to the right of the arrow and looks like a box with a small plus sign at the bottom right corner of the box. Click it and draw a selection around the keyframess that you want to adjust. It will select all the superimposed keyframes and let you adjust the x,y and z scale values simultaneously. For more info type tramsform keyframe tool in the search box under help menu.

Instead of clicking the keyframe you want to manipulate, try dragging a small rectangle around it - that selects the same point on all three curves at once. You can then move all three at once. You can also change the interpolition (to Bezier for example) for all three at once and modify the shape of the curves by dragging one of the handles. You can even separate one point from the others (Y scale for example), select all three points and modify the control point handle of one and all three will change.

@alex4d