I would say it is a combination of set in seams and partial seam method. You can google partial seams to find tutes on it. It would have a ton of them. There may be an easier method but you would most likely have to purchase TocoolTshirtsquilts book to get it. I certainly don't see an easy way to do it with that layout.
Like pocoellie, I utilized sashing strips to get my different size blocks to work. I also cut them in sizes evenly divisible by 3 which also helped. Here is mine:
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