Your quilt is spectacular and OUTSTANDING!
For the three borders, try sewing the three pieces together FIRST, before putting them on the quilt, and then miter the corners on the quilt. This keeps the seams a lot straighter and you can sew them together without the whole weight of the quilt to deal with. When you get to the point of sewing the three borders (joined) onto the quilt, you will have a wider, more stable piece to work with. It works like a charm.
You can look up how to miter the corners online if you don't know how. Just sew your borders to their respective sides and stop 1/4" from the ends. Then you will make the mitered seams.