Let $P(x) = ax^3 + bx^2 + cx + d$. If the graph of $P(x)$ passes through the origin and the roots of $P(x)$ are $r_1 = 1$, $r_2 = -2$, and $r_3 = 3$, which of the following expressions represents $P(x)$ for some constant $a \neq 0$?