When cooking rice you use 1 cup of water per each cup of the dry rice you're cooking.
So if you have 3 cups of dry rice that you're cooking then you would need 3 cups of water to add to the 3 cups of rice that you're cooking.
So it takes 3 cups of water to cook 3 cups of rice.
When cooking rice also make sure you stir the rice and watch the rice if you're cooking it on a stove to avoid the water from boiling over in the pot.
You don't want to add too much water to the pot of rice because it could boil over.
Get the pot of water and rice up to boiling and then turn it down to a simmer and cook the rice for 5 or 10 minutes or until the water is absorbed by the rice and the rice becomes soft.
If the rice is still a bit hard then you should add some more water and continue cooking the rice for another few minutes or so.