Not sure about ending in 3,6 or 9, but if you are making your own panel, it will be much easier to shoot for a size where both the horizontal and vertical measurements are divisible by the same whole number plus 1/2".
So for instance, a panel that is 33 1/2 by 42 1/2. If you subtract a half inch from both measurements, you come up with 33 and 42, which are both divisible by 3. You can border the panel with 3" blocks in both directions and have it come out with complete blocks. That means the blocks would be cut at 3.5", and finish at 3". the extra half inch in your panel is for the final seam allowance.