For the sushi rice, wash the rice at least 3 times or until the water runs clear. Bring the washed rice to a boil in a pot with the 3 cups of water.
Then turn the heat to the minimum and let it sit for 20 minutes. Remove from heat and let stand covered for another 20 minutes.
In a saucepan, slowly heat the vinegar, mirin and sugar. It should not be boiling but hot. Mix the liquid with the rice.
Transfer the sushi rice to a bowl and let sit, covered, for another 30 minutes.