In a large, heavy saucepan, combine the potatoes, carrots, onion, and salt. Pour in enough water to cover the vegetables and bring to a boil over medium-high heat. Continue boiling until the potatoes are soft enough to be speared with a fork, about 4 to 5 minutes. Remove from the heat and strain the vegetables through a fine mesh sieve. Reserve the liquid as vegetable stock for another use, or discard.