Remember to add a seam allowance to each side of the center section width. Add a seam allowance and hem allowance to the width measurement of the side sections.
Adding piping to the seams is an opportunity to add contrast and makes the seams look like they have a decorative purpose.
If a full width of fabric will be needed for each section, your fabric requirement will be three times the desired length of the bed spread PLUS an allowance for a hem on each end.
To keep the end corners from sagging on the floor, round them off, using a compass or plate before you hem the bottom.