Inkscape in Six Steps

3D Box

First, use the pen tool to draw a big flat triangle. Make sure the longest edge is horizonal. That line will become our horizon line and the two points are the vanishing points.
Draw two more lines from the vanishing points to one of the edges. These two lines and the two edges of the triangle form the bottom of our 3D box.
Select all and group them. Make a duplicate by clicking on the button bar. While the duplicate is still selected, scale its height as show in the picture. This is the top of our box.
Now you have all eight corners of the box, as show in figure, coded 1-8. Use the pen tool to connect 1-2-3-4, 1-4-6-7, and 3-4-6-5. We now have the three faces of the box.
Delete the two triangles and leave only the box. You can see the box is almost there, but the three faces are not perfectly aligned and there are gaps and overlaps between them.
At this step, do not worry about the corner of the faces does not snap to each other. We will deal with that issue in the next step.
Select all three faces and do a path combine. You can do that through Menu, Path, Combine, or you can do it by Ctrl-K. Now select the combined path, swich to node tool . Then click the alignment button .
Now we see the alignment dialog shows the node alignment buttons. Use horizon and vertical alignment on corner 1, 3, 4 and 6 to stitch the three faces together.
Finally, un-combine the three faces and put some shade on them. It is here, our perfectly perspective 3D box!