In the case of a martini pendant, there is no need for balance, of course since it is hanging from a chain. I assume that since there is no need for balance, the goal of the three prongs is just to reduce the amount of diamond covered by prong and to give a unique look to the stone that is not provided by the four or six prong setting? At any rate, I like it! Just as I like the martini set earrings, which I do not own! Congratulations on a lovely choice!
PS-Do you have martini style earrings?