The logical way to get all the starter Pokémon in Sword and Shield is to simply trade them with other players. This involves a few steps, and you also need to know people who play the game and have access to the internet. The first thing you need to do is unlock the Comm-Link. You need to play through the story and will know it has been unlocked when a blue Comm Links icon appears in the bottom left corner of the screen. Hit Y to open the Comm Links menu to begin trading Pokemon. You can do this either offline or online.
Link Trade will allow you to trade Pokemon with anyone on your Friends List or in the local area. If your friends are not near you, using the + button in the Trade Menu to connect to the internet and trade with them that way.
There is a way to get all the starters even if you are stuck on your own, and don’t know people who play Sword and Shield. To do this, you will need Pokémon Home. You can download the free version from the Nintendo eShop for your switch. This will then allow you to transfer Pokémon from Sword and Shield to Pokémon Home, and back again.
Now, all you need to do is start up a game of Sword and Shield and pick one of the starters, then move it to Pokémon Home. Do this for each starter, and you will be able to move them all back to whichever save file you choose to keep playing as in Sword and Shield.
It won’t take long, as you pick your starters quite early in the game, and you will then be able to play as, and level up, all three starters in the same game.