This looks great, but I would put the white in the middle. It's the outside rails that make the stairstep design, and I would personally prefer not to have the white making the stairsteps. It will look great either way, but that's just my personal preference. I love the inspiration print.
As for pressing, I would put all three strips together, then press. And I agree with the others. Take an average width of your strip sets, and use that measurement to cut your squares.
Darren