Nature may abhor a vacuum, but she’s not keep on even numbers either. Almost all plants have odd numbers of leaves and/or flower parts, yes, there are exceptions like the dogwood tree and the elusive 4 leaf clover. Odd numbers of objects in an image are more pleasing. When even numbers of objects are present, our brains expect balance and symmetry.
Three ducks are easier to balance than two. They may be harder to juggle though.