Normality is essentially concentration times the number of equivalent solutes. There are 3 equivalents in H3PO4. Therefore, the molar concentration is simply 0.30 N/3 = 0.10 M. This is the concentration before dilution though. If they are asking you to calculate the final concentration, you need to calculate moles of H3PO4 (0.10M x 0.04L) and then divide by the final volume of solvent in the solution.