Three-stone diamond rings are the most popular diamond anniversary rings. The three-stone "past, present, future" rings has become an increasingly popular choice for brides-to-be. Sales of the 3-stone diamond rings where a larger diamond is flanked by slightly smaller ones, now account for approximately 10 percent of diamond rings sold. There really are no rules for what shape of diamonds can be used and what size is best. The 3-stone ring is truly one piece of diamond jewelry where you need to rely on what looks best to your eyes.