Over 100 Pokémon have been added into Pokémon Sword and Shield with the new The Crown Tundra expansion, and included in that number is another set of regional starters.
All three starter Pokémon from the Hoenn region are now available to obtain in Sword and Shield, though you will need to put in some leg work to get them on your team.
As of now, the only way to capture Torchic, Treecko, and Mudkip is to run into some variation of their evolutionary line while participating in a Dynamax Adventure. It appears that most players will encounter the middle evolutions, Combusken, Grovyle, and Marshtomp in their dungeon runs.
You can also find the fully evolved starters, Blaziken, Sceptile, and Swampert available as Rental Pokémon when you first set out on a Dynamax Adventure.
There are likely going to be a few other ways to obtain the gen three starters, but for now, just keep grinding out the Max Raid Battles and you can eventually find them for yourself.