I agree with katier and arleners but the simplest way to do this is measure quilt side.... i.e., 86.5.... and 3" fin sq..... 86.5/3 = 29 + just for the side, no corners allowed yet. so you need one more for each corner overlap...now 31... NOW make your life easy...add 1... 31 x 3" = 94" :Width = 74.5/3 =25 (corners are already accounted for, but you still have to add the extra 1.... 26 = 78"
Now sew these into the open square they will make... it will be 31 Three " squares x 26 three " squares and will measure 94 x 78... lay flat, put quilt in center....measure the space between quilt and squares and add 2 seams and that will be the cut width of your inner border... when piecing borders, make them first and then put in the filler border... make this filler match your quilt background and your entire center will appear to float...