Quick Biology question. Can anyone clarify this for me, i am quite confused. I understand the question except for what they mean by a multiple of 3 or not a multiple of 3. What does it mean by multiple of 3 or what is it referring to? Thank you very much!!

An enzymatic hydrolysis of fructose-1-p, fructose-1-p(aq) + h2o(l) - fructose (aq) + pi (aq) was allowed to proceed to equilibrium at 25°c. the original concentration of fructose-1-p was 0.2 m, but when the system had reached equilibrium, the concentration of fructose-1-p was only 6.52 x 10^-5 m. calculate the equilibrium constant for this reaction and the free energy of hydrolysis of fructose-1-p.