48x57 = 2,736 sq inches in your pieced top
48+5=53 57+5=62
53x62=3,286 sq inches with 2.5" added for each side
3 yards = 3x36= 108 inches length
108x42= 4,536 sq inches of backing material
The fun thing is now, how to cut the fabric. I go simplistic. I would use the 2 yard long, 72" piece lengthwise across the 62 inch. (You should have an extra 10 inches.) Then I would take the 1 yard piece and cut it in half, as in 2 pieces that are 36" long x 22". Sew the 36" length together, and now you have a piece that is almost 2 yards long x 22" wide to sew to the original 2 yard piece. After sewing this new pieced backing fabric should be about 71" x 64". Now you can trim to the size you want to cover your pieced top which is 53"x62" which includes the 2.5" on each side.
Check my math carefully before you cut. I'm a quilter and I am not perfect.