$H _3PO _3$ is a dibasic acid. In its structure, one oxygen atom is bonded to $P-$ atom via a double bond. $2$ $O-$atoms are bonded to $H-$atoms causing acidity. The third $H$ is directly attached to $P-$ atom responsible for its reducing behavior.
In $NaH _2PO _3$, one acidic $H$ is replaced by $Na$. Only one $H$ is left which causes the acidic behavior in the salt and hence, will react with one mole of base.