Cut three pieces of 2-by-4 lumber to 45 inches each for each platform you intend to build.
Lay a sheet of 3/4-inch plywood on a pair of saw horses. Use a tape measure and chalk line to make a line down the center of the sheet, running crosswise the short way, at 48 inches from each end.
Drill 1/8-inch pilot holes along all four edges, 3/4 inches in every 10 inches and along the chalk line you marked.
Clamp an 8-foot 2-by-4 to each long edge, flush at the ends and along the long edge, with the 2-by-4 on edge. Drive one 3-inch treated deck screw through each pilot hole into the edge of the 2-by-4 with a cordless drill. Attach two of the 45-inch pieces at either end in the same way.
Position the remaining 45-inch piece in the center of the plywood, on edge, and drive screws through the pilot holes into its edge. Drive two screws into each end of this board, through the outside 2-by-4s and in each corner to hold them together.
Cut two pieces of 3/4-inch thick plywood 10 1/2-by-12 inches and two pieces 12-by-12 inches, using a table saw. Use a square to draw a 3 1/2-inch square centered along one 10 1/2-inch edge of each of the smaller pieces and along one 12-inch edge of each 12-by-12 piece.
Cut the squares out with a jigsaw to form a 3 1/2-inch notch in the end of each piece.
Apply wood glue along the two 12-inch edges of each smaller piece. Stand one of them on its narrow end with the notch up. Nail a wider piece to it, with the notch up, and overlapping the smaller piece 3/4 inch. So that the edge is flush with the outside face of the smaller piece, use one 1 1/2-inch pin nail every 2 inches.
Nail the other two pieces together in the same way and fit the two halves together to form a 12-by-12 square column, 12 inches high.
Fit one column under every outside corner of each platform.
Use two columns to join two platforms, with the edges of the platforms fitted together in the top notches of the columns to hold them
Use one column under each junction of four platforms. Lock them together with the corner of each platform by fitting them into the column notches on the two facing sides so that the points of the platforms all are in the center of the column.