What is the equivalent weight of H3PO3 in the following disproportionation reaction:-

H3PO$\to$H3PO4 + PH3

(1) $\frac{M}{6}$

(2) $\frac{M}{2}$

(3) $\frac{2M}{3}\phantom{\rule{0ex}{0ex}}$

(4) $\frac{M}{3}$