I find that if I am going to add multiple borders, it's better to sew all the borders together and remeasure the quilt top in three areas and cut the sewn together borders to match the averaged quilt side measurement. If you sewed the borders on one at a time, they do tend to expand.
At this point you have two options, take off the borders and fix the slack (the quilt edge may now be distorted and be harder to fix) or try to work in the extra with dense quilting if you can. If you have quilted the center and not the borders, this will have caused some of the problem. The quilted center shrinks up as you quilt it. Good luck!