The concepts use to solve this problem are shown below, but the actual answer is not clear based on the results.
Your chemical equation shows that the mole ratio of the reactants is
1 H3X to 3NaOH.
You have reacted 18 mL of .500M NaOH which is
18/1000 L x .500mol/L = .009mol of OH-
That would indicate that the acid needs to contain .009mol of H+
Since the acid is triprotic, there will be .009/3 = .003 mol of the acid.
That means .240g of the acid is .003 mol, so its molar mass is .240g/.003mol or 80g/mol.
Using the molar mass numbers for some possible acids leads to
H3PO3 (phosphorous acid) with a molar mass of 82g/mol.
That matches the implied answer in the last part of the problem.
HERE IS THE PROBLEM - H3PO3 is NOT triprotic! It is diprotic.
I haven't found a triprotic acid with a molar mass close to 80g/mol
H3PO4 is triprotic, but its molar mass is 98g/mol which is too much.
I hope another tutor will find a better match to give a better answer.