I'm not sure at all about these two, Beth. The three small pendants of Princess Helena's brooch seem to have a shape quite different from the three dangling diamonds of the piece the Duchess of Gloucester is wearing, but that might be an optical illusion due to Princess Helena's rather prominent bosom. And of course the three pendants could have been exchanged at one time or another for the diamonds we see today.
It was a mere idea that if(!) Queen Mary indeed acquired one piece of jewellery which once belonged to Princess Helena, it might have been not the only one.