saying 3% generally doesn't mean a whole lot to someone---pre-wash your fabrics---shrinkage depends on a number of things- including how densely it is quilted- the backing, (generally) if you have prewashed your fabrics, you quilt an all over design- and wash the quilt in cool water, gently- tumble dry low/medium heat; the shrinkage will be enough to give the quilt that (crumply/antique(y) look....there really wont be much length/width noticable difference...3% would be 3" in 100" ...
i've been quilting for years- have never pre-washed a batting- have never had a quilt shrink so much it made a difference in how it fit the bed it was made for.