in H3PO3, check the oxidation state of P…… it is +3, which can be further oxidised to higher oxidation state. Thus it can act as reducing agent. here we can say that h3po2 will act as stronger reducing agent for the same reason.(oxidation stat +1). read more
Obviously H3PO3 is more reducing agent because in H3PO3,P in +3 oxidation state So it can be oxidised to max +5 oxidation state,So it is a good reducing agent. On the other hand in H3PO4 P in +5 oxidation state So it is in max oxidation state So it can not be further oxidised so it can not reduce other so it is not a reducing agent. read more
in H3PO2 and H3PO3 , 2 hydrogen atoms and 1 hydrogen atom respectively are bonded to phoshorous atom. But in the case of H3 PO4 , there is no hydrogen atom bonded to phosphorous atom. So, the oxidation state is lower in H3PO2 in comperision to H3PO4 so it is not a reducing agent. read more