Question: how much water to cook 1/3 cup lentils?
Answer: Lentils to do not require soaking like other pulses.
Rinse your lentils with fresh water before boiling to remove any dust or debris.
Cook on a stovetop, using 3 cups of liquid (water, stock, etc) to 1 cup of dry lentils. ... 
Bring to a boil, cover tightly, reduce heat and simmer until they are tender.