Well, isn't that a happy little question! To find the volume of topsoil used, we first need to convert the depth from inches to yards. Since 1 yard is equal to 36 inches, 3 inches is 3/36 = 1/12 yards. Then, we multiply the area of the garden by the depth of the topsoil to find the volume. So, 24 yards (area) multiplied by 1/12 yards (depth) equals 2 cubic yards of topsoil used. Happy gardening!