Heat water to boiling in a medium pot. Add rice and bring to a simmer for 10 minutes.
In the meantime, pare the sweet potatoes. Using a hand-held peeler or a food processor, shred 2 of the sweet potatoes, to make 1 cup lightly packed. Cut the remaining sweet potato into ½-inch chunks.
Stir the shredded sweet potatoes into the rice and add the chunks on top of the rice. Cover and simmer for another 20 minutes, until the shredded sweet potato has dissolved into the porridge. Stir very occasionally to prevent sticking at the bottom of the pot. Serve hot.
This is the ultimate comfort food -- what my mother prepares, apparently, when she is cooking for one. This makes a good substitute for rice at a meal, or can be a standalone breakfast. You can always make several servings and refrigerate it. To reheat refrigerated congee, add small amounts of boiling water and heat over medium heat while stirring frequently to avoid sticking and burning.