While the formula you used is valid when working with monoprotic acids and bases, it is more difficult to use it for polyprotic acids and bases. A more general approach would be to calculate the number of moles of NaOH, then convert it to the number of moles of acid. In your example, 1 mole of NaOH reacts with only 1/3 mole of citric acid. Once you have that, it's easy to calculate mass and concentration.