When I'm putting the finishing touches on my skirts they almost always have a 'three' theme.
Whether it's three different colours, three pieces of applique, three panels of fabric or three rows of stitching.
Very often... it's three buttons. Generally sewn together in a little cluster. It always seems just the right amount. Not too much. Just enough.
I usually put images in a set of three too. To me there's always something visually pleasing about seeing three things grouped together. Even though it's an odd number... it always looks balanced. I think so, anyway.