Answer
$PCl_3(l)+3H_2O(l)\rightarrow H_3PO_3(aq)+3HCl(aq)$
Work Step by Step
Phosphorous trichloride is $PCl_3(l)$. Water is $H_2O$. Phosphorous acid is $H_3PO_3$ and hyrochloric acid is $HCl$,
$PCl_3(l)+H_2O(l)\rightarrow H_3PO_3(aq)+HCl(aq)$
Add a coefficient of 3 to $HCl$ to balance $C$.
Add a coefficient of 3 to $H_2O$ to balance $H$ and $O$.
$PCl_3(l)+3H_2O(l)\rightarrow H_3PO_3(aq)+3HCl(aq)$
