Answer
$f(x)=\dfrac{1}{2}(x+1)(x-2)(x-3)$
Work Step by Step
Here, we have $f(x)=a (x+1)(x-2)(x-3)$
When $x=0$, then we have $3=a (0+1)(0-2)(0-3)$
or, $6a=3$
This gives: $a=\dfrac{1}{2}$
Thus, we have $f(x)=\dfrac{1}{2}(x+1)(x-2)(x-3)$