That's how I would do it, and that's how I've done it in the past. For a total of 3" wide sashing, use strips cut at 2". They will finish at 1.5 each side of the block, which will give you 3".
Another thought - with white or light-colored fabrics, the seam lines will be more visible. If you want to disguise this, use a white-on-white or some other kind of print.