In HNO2, Nitrogen has oxidation state +3, nitrogen has maximum oxidation state +5 and minimum −3, hence nitrous acid can act both as oxidizing and reducing agent.
Sulphur has maximum oxidation state +6 and minimum is −2, in H2SO3 sulphur has oxidation state +4, hence it can act as both oxidizing and reducing agent.
For oxygen maximum oxidation state is +2 and minimum is −2, in H2O2 Oxygen has oxidation state −1, hence it can act bot as oxidizing and reducing agent.
As in H3PO4, Phosphorous is present in it’s maximum oxidation state i.e., +5, hence it cannot act as reducing agent.