I did a similar pieced sashing. I sewed 3 WOF strips, then sub cut to the length of the blocks. Mine were also cut at 1.5", so the sashing finished at 3". I pieced 9 patches for the cornerstones. Well worth the effort. Looking at the picture, the sashing looks like it finishes about the same size as the HSTs in the corner of the blocks. That might be a bit smaller than 3"