OK... here is another idea. What if I use all three fabrics, in diagonal stripes? For instance, sash the top left corner block with blue, then the block below it, and the one to the right of it in green. Next, the 3rd from the left on the top row, the 2nd from the left on the 2nd row down, and the left corner block on the 3rd row down would all be sashed in red. Thus, the 4th on top, 3rd on 2nd row, 2nd on 3rd row, and first block on left of 4th row down would all be sashed in blue. The pattern would continue this way.
I would put a border of each color around the edges (with strips of the uncut fabric top and bottom mixed in). Does this make sense, and if so, what do you think?