If you mean to do something in your creative project, do it 3 times so people know you did it on purpose. It brings harmony and unity to your effort. This is true in art, in gardening, or in designing anything.
Here's what reminded me of this "rule" from my college typography class: As I design with stained glass I find myself always looking for a pattern to subtly bring forward. Not too much or it would be just plain boring.
In the planning the panel "Celebration", I introduced the yellow triangles to break up the monotony of all the squares and rectangles, I purposely included smaller green triangles and orange diagonal lines, too. I didn't just throw any ole triangle, but echoed colors used previously.
The small squares in "Celebration" were added to bring variety to the size - which creates tension and balance but notice all the colors are not the same. Nor are they all on the same row, but they are all organized toward the middle and the small squares are the same size. The type of glass is even similar.
Repeat the shape 3 times, the color 3 times, or the line 3 times. Whatever you want to emphasize. It doesn't have to be exactly the same for repetition, in fact it would start to look canned and contrived.