Cut each of the 3 shelves in half - you now have six lengths of shelving, each is 3 feet long.
Each shelf piece has one long edge which is bent, and one long edge that is "flat" (not bent). To assemble the cage, the shelf places are turned on their ends, and the long edges are joined by zip ties to form the main body of the cage. Use 2 pieces each for the front and back, and one piece for each side. The bent sides of each shelf pieces all meet at the corners. The top of the picture above shows how the corners come together. Two corners will look like "A" where one bent edge and one flat edge meet, while the other two corners will look like "b" where 2 bent edges come together.
At the middle of the front and back the flat sides of two shelves are joined. Initially, the sides of the cage are a bit unsteady as they are able to hinge, but this will be stabilized as the rest of the cage is assembled.