My girlfriend who quilted for years used whatever she had. One time it was an old set of drapes that was washable fabric. Another time she used a blanket that was slightly torn. She mended the blanket by hand stitching it back together so that it laid flat. If all the fabrics have been prewashed before you put together the 3 layers you should be okay. If 2 are prewashed and another not, they might shrink differently and you might not like the end result. A sheet might make a good center, I know it makes a good backing. I use them all the time.