An algebra problem by Pranay Kr

$\large x^3-6x^2+ax+a=0$

Let $a$ be a real number, and $r_1$ , $r_2$ , $r_3$ the roots of the equation above. It is known that $(r_1-3)^3+(r_2-3)^3+(r_3-3)^3=0$ . Compute $a$ .