What you need:
1. Line the 3 boxes up one in front of the other
2. On the side of the boxes, make a mark 9 inches from the bottom of the last box and 5 inches from the bottom of the first box. Do this on both sides of the box.
3. On each side, connect the 9 inch mark to the 5 inch mark, drawing a diagonal line across the sides of all 3 boxes.
4. Take away the first box and draw a line across the front of the box that connects the lower ends of the diagonal line you have already drawn on the side. On the back of the box, connect the higher ends of the diagonal line. Do this for all 3 boxes.
5. Cut all boxes along the lines you have drawn and remove the tops.
6. Line up all 3 boxes again, and tape them together.
7. Decorate however you like!