Zod's crest can't be a 3, cos it's kind of a double standard. If you're using part of the outer shield to make Zod's "3" crest, why not use Superman's as well? It's more likely designed to be a sickle/Omega (to a lesser extent, although I liked the explanation of Z being the last letter of the english alphabet so they're doing a last letter of a greek alphabet association) thing.