In generation three not all possible pokemon are legal. For example 6iv can only exist with some natures, its been a very long time since I've messed with nature legality but if I remember correctly only modest and timid can exist with all 6 31 ivs in generation 3. I tried to look it up but was unable to find anything. If you're also trying to make it shiny that would be more difficult since it would likely require you also use an rng assistant to find extra information you need but I think the only good rng assistant for XD is in Japanese. Sorry I couldn't be more helpful.