You aren't cutting it into even thirds.

It looks like you have divided it into fourths and cut a 1/4 and 3/4 leaving the middle block and the middle strips twice the size of the outside corners.

To find the lines to cut even thirds, take your block diameter, divide by three and cut at that number and twice that number. For example if the large pinwheel was 12" across it would be 12/3=4" so you'd cut at 4" and at double that 8" (or 4" from the opposite side) this should give you 9 even sized squares.

Edit: KerriS beat me to it by 2 minutes.