Question

H3PO2 and H3PO3 act as good reducing agent while H3PO4 does not.

Answer 1

because H3PO4 does not have P-H bond

Answer 2

In these compounds the h atoms bonded to P atoms are responsible for reducing power and those bonded to O are responsible for the acidic character of these compounds and H3PO4 does not have any P-H bond .