The Rule of 3 recommends grouping 3 items together based on brain science that proves there is something about odd numbers that makes people take notice, and look more closely. Remember that even though it is called the Rule of 3, other odd number combinations can work just as well. It depends on how many items you need to display to show the variety of your selection, and what makes an attractive arrangement.
Balance symmetry and asymmetry:
Human brains like symmetry up to a point, but too much becomes boring. That is why pyramid layouts are so popular where you put a taller item in between two smaller items so customers look at the focal point first, and then work their way around the whole display seeing everything that you are presenting.
Like any rule, there are times when you want to bend or break the conventions. If you have an item that is exceptionally large, valuable or interesting, it is usually better to let it stand alone with a display case of its own.