Not going to be easy using a charm pack, as the charms are 5 inches. Not easily divisible by 3, to add the 9 patch.
5 inches divided by 3 is 1.67 + .25 for the seam allowance + 1.92. So your strips for your 9 patch would be cut just shy of 2 inches. Not sure how many 8th's of an inch this would be.