I see Harry, Snape and Voldemort as a trio. I think the trios (also Harry, Ron and Hermione) are supposed to be different sides of one whole person. The twins are probably two sides of a coin or sometimes just variations of a theme?

Sirius and Bellatrix are mirror images

Draco Regulus: Sensitive little pure blood boys joining the Death Eaters and changing sides because they have a conscience. This is another reason why Draco has to survive when Regulus died, because this war is going to have a happier ending.