I think this is the best set up to cut your 3" and 2½" inches strips, without loosing too much wood.
First - Cut your two eight feet 3 inches strips.
Second - Cut your four strips of 3 inches cross members. You will cut these strips at 47 inches from each ends, until you have seven equal pieces. The last strips, (8th piece) cut it at 35 inches. This strip will be one of the leg braces.
Third - Lay down your plywood flat on the floor so that your eight feet is from left to right. From the left, measure 40 inches. Cut on your 40 inches line. Put the right side of the plywood aside.
Fourth - Cut your 40 inches long plywood into four, three inches strips (for front and back legs) and than 4, 2½ inches strips (for side legs).
Fifth - Cut the final 3 inches strip at 35 inches long for the last leg brace.
Final result, you should have six pieces of scrap wood and a nice piece of wood remaining appoximatly 55 7/8" X 29 1/4" X 1/2".
Keep all your scrap pieces of wood (mark with a red X), they could come to be handy
D'ont forget, your seven cross members are to be placed 16" center apart.This table top should be for a tight HO layout. I say a tight layout, because Most of the HO plan tracks come close to the edge of the table. For a better result, the table top for an HO layout plan would be a 9' X 5' table top for the simple reason in case of derailment, your train will not fall on the floor, but on its side on the table top.