(N/A) The shapes of the molecules are determined by the $VSEPR$ theory,considering both bonding and lone pairs of electrons:
$1$. $H_2O$: The central oxygen atom has $2$ bond pairs and $2$ lone pairs. The geometry is tetrahedral,but the shape is bent (or $V$-shaped) with a bond angle of $104.5^{\circ}$.
$2$. $NH_3$: The central nitrogen atom has $3$ bond pairs and $1$ lone pair. The geometry is tetrahedral,but the shape is trigonal pyramidal with a bond angle of $107^{\circ}$.
$3$. $SO_2$: The central sulfur atom has $2$ bond pairs (double bonds) and $1$ lone pair. The shape is bent (or $V$-shaped) with a bond angle of $119.5^{\circ}$.