Work done by $A$ in $1$ hour =$\frac{1}{4}$

Work done by $B$ and $C$ in $1$ hour=$\frac{1}{3}$

Work done by $A$ and $C$ in $1$ hour=$\frac{1}{2}$

Work done by $A$ ,$B$ and $C$ in $1$ hour=$\frac{1}{4} +\frac{1}{3}=\frac{7}{12}$

Work done by $B$ in $1$ hour $=$ (Work done by $A$ ,$B$ and $C$ in $1$ hour)$-$(Work done by $A$ and $C$ in $1$ hour)

$=$$\frac{7}{12} - \frac{1}{2} = \frac{1}{12}$

$B$ alone will take time to complete his work =12 Hours