Solve the molarity equation M=n/V for NaOH to find the amount of moles that were reacted. Take this n and note that in the balanced chemical equation, 3 moles of NaOH react with 1 mole of H3PO4. So take the n you solved for, divide by 3, and use this amount of moles and molarity of phosphoric acid to solve for the amount of volume that is required to maintain the concentration.