I think it's possible to RNG for them. Never tried myself.
This, I think is the only way to actively alter your probabilities in Generation 3 games. The Masuda method didn't exist until Gen 4, and the ability to pass shininess down by breeding was removed after Gen 2 as part of the data overhaul.
All above is completely true and accurate, unless it isn't. 3DS FC: 1075-2072-0394 (Ems) | Pokemon X: Mallory