Instead of anchoring to left and right, change the anchor point to be 0.1667 and (1 - 1.667), respectively (assuming the size is actually 33%, not 30%? If it's 30% then it should be 15% and 85%, respectively).
Your anchor should also be on an empty game object, not on the sprite. The stretch script can remain on the sprite.
Set up like this, you can then add a label to your game object, and it too will be centered within your 30% cell.