Originally Posted by GingerK

That is a good question. First of all your math is off. If you take 2 one yard pieces (your first calculation of 42+42) get a piece that is 84x36 inches. If you take those same two yard pieces (your second calculation 36+36 ) you get a piece that is 72x42 inches.

In each case, you have to add another strip to get the size you need. Do the instructions say how to piece the backing? If it was me and I had 3 yards of backing, I would cut the length that I need--say 76 inches. I now have a piece 76 inches long by 42 inches wide. I would take the remaining 32 inches xWOF and divide it into 3 equal widths (in this case, 14 inches) and sew them together end to end. Then I would cut that long strip to 76 inches and add it to my first piece. I would now have a backing that is 76 inches long by 56 inches wide. A bit tight size wise but if it is not going to a longarmer, you should be able to manage with a lot of basting.