There's no way to increase shiny chances beyond the 1/8192 probability in Gen 3, unless you use RNG manipulation. That's why shinies in Gen 3 are so rare and valuable. If you really want to shiny hunt, then you can do what I see a lot of people on YouTube do, which is put multiple GBAs running Pokemon side by side, and shiny hunt on all of them at the same time. It will still take weeks to months, but will be faster than just hunting on one game at a time.