How to get Shiny Aron, Lairon and Aggron
As I said above, the chances of catching Shining Aron have been increased by the Hoenn Celebration event. While we don't know the exact number yet, it should be significantly better than the usual 450 you get in standard encounters with no active event. And on top of that, Aron will be easier to get in a number of different ways, making it a great time to focus on hunting your own Shiny Aron.
The best way to get one is in the wild, as Aron is one of the Pokémon whose spawn rate was enhanced during the Hoenn Celebration Event. It's also part of the Hoenn Collection Challenge, so you should already be working to catch one. Using incense to activate a decoy on a Poké Stop will give you a higher chance of spawning in the wild. On top of that, there are two ways to try and get a Shiny Aron while the Hoenn Celebration Event is taking place.
Aron will also be added to the 5km Egg Pool during the event, so try to get some eggs before the celebration ends. In addition, from January 19 to 24, special field research tasks will be assigned to Poké Stops. “Power up Pokémon 5 Times” will reward you with an encounter with Aron, with the chance that she is brilliant either way.
Focus on these three things, catching one in the wild, getting one in a 5km egg, or completing the “Power up Pokémon 5 Times” field research task and this is how to get Shiny Aron during the Hoenn celebration event in Pokémon GO. And if you want Shiny Lairon and Aggron, you just need to evolve it, so hopefully you end up with three in total before the increased chances end.