Reducing agents are those molecules which can donate electrons...

In H_3PO_2H_3PO_3 the donating elctrons are Hydrogen atoms which are bonded to phosphorus atom

In case of H_3PO_2 there are two hydrogen bonded to P atom
But in case of H_3PO_3 there are only one hydrogen bonded to P atom

Therefore H_3PO_2 has more donating electrons than H_3PO_3.

Therefore H_3PO_2 is stronger reducing agent than H_3PO_3.