Two things come to mind: by replacing the circle/dots fabric, you are losing the light value contrast, and you are also losing contrast of pattern/scale.
So, if you want to replace the circle/dots fabric in order to get a third color in there you might go through your stash and look for very light valued fabrics in colors other than the greens and reds that you've already got in there.
On the other hand, you could keep the circle/dots light valued fabric and replace the pink flowered fabric strips that are currently beside it with the orange flowered fabric (which gets you the third color and is similar in scale, value and type of print to the pink flowered fabric that would be replaced). Or, instead of the orange flowered fabric, look for another medium valued fabric in your stash that is a different color, or different type of pattern, or maybe a solid.
Rob