When I first started making jewellery, I made life really difficult for myself by teaching myself how to make bezel settings of all shapes. I’m glad I did, as I can now set any shape of stone, but it was a steep learning curve, particularly as my tool set was pretty basic - and still is! I used to use a triangular mandril to make the basic shape, but a little larger than the stone’s sides if measured corner to corner in a straight line. Then I used a round mandril to gently put a curve into all three sides. I repeated the process to make an inner (step) bezel for the stone to sit. As I say, I made life difficult for myself! I’ll be interested to see some more answers to your question.