Oxalis stricta and tristyly
This is Yellow Wood Sorrel, or Oxalis stricta. It is a member of the Oxalidaceae family, and has tristyly. This means there are three possible orientations of its stamens and pistils (the reproductive parts).
Here is an example of tristyly:
The green pieces are pistils, and the yellow-topped ones stamens. The pistils can be the shortest (left), the middle height (center) or the tallest (right). All three occur in the same species.