how do I thin the plants (veggie sprouts) in my garden?

Thanks to all the wonderful flutherites out there who sent ideas for how I could find local community gardens, I now have my own plot. I planted carrots, swiss chard, and kale. I have little sprouts growing. I’ve been told I am supposed to thin them. How big should the sprouts get before I thin them? I planted three seeds in each little hole, so I have three little sprouts growing close to each other. How do I thin them without tramautizing the root systems? Do I just pull two of them right out as if they were weeds? Clearly, I’ve never gardened before…