When you begin your playthrough of Pokémon Sword and Shield, you will need to decide who your starter Pokémon will be. You can pick from Grookey, Sobble, and Scorbunny. Thankfully, you can also get the ones you don’t choose by taking advantage of a couple of the game’s features.
Your currency for getting the other two starters will be your starter, as the idea is to trade with people who are looking for whatever Pokémon you picked in exchange for theirs. Because you can’t just trade away your starter if you want to have all three, you will need to get your hands on a Ditto.
Ditto can breed with your starter Pokémon, producing Pokémon Eggs that will give another Pokémon the same species as your starter. After that, it is merely a matter of finding a player who is looking for the Pokémon you have and is also breeding their spare starters for trade.
To breed Ditto and your starter, head to the Nursery on Route 5. Leave your Ditto and starter Pokémon with them for half an hour, and then return. If it has resulted in an egg, you add the egg to your party and then play the game. You need to build up travel distances to hatch eggs, so an easy way to do it is by using the fastest method in the game.
And that’s it, all that remains is to try and find a player who is looking to trade. I would suggest starting in the comments section of the video linked above. Hopefully, it will become a nice hub for people are looking to trade starters with each other.