You can attach mitred borders separately or all as one. If you want to attach them all as one, you first sew them together like a strip set. Then treat that strip set as one piece of fabric. You would measure your quilt width, add the width of the borders (times two, because they'll be on both sides), add a couple of inches for insurance, and then cut two strip sets for that width. Do the same for the quilt length, cutting two strip sets.
If you want to add all 3 borders together, then for your quilt, if the center is 60x80.5 and the 3 borders together total 11 (3.5+2.5+5, and I am assuming these are FINISHED sizes, not the unfinished sizes), then two of your strip sets should measure 60 + 2*11 +2 = 84" and the other two strip sets should measure 80.5+2*11+2=104.5". (You should check my math, because I haven't.)