#### Answer

$3x(x+1)(x-1)$

#### Work Step by Step

Since $3x$ is common to all the terms in the expression, we obtain:
$3x^{3}-3x$
=$3x(x^{2}-1)$
Now, we use the relation $a^{2}-b^{2}=(a+b)(a-b)$ to simplify:
$3x(x^{2}-1)$
=$3x(x^{2}-1^{2})$
=$3x(x+1)(x-1)$