It can stand for whatever you want. That's the beauty of the Triforce, it can mean many things to many people. Legend has it the three pieces of the Trifoce represented wisdom, power, and courage. It's possible the S, P, and D stand for names of the people who hold those three pieces of the Triforce. Just imagine what would happen if those three people were brought together and joined forces, literally. The world would be theirs.