How to write the distribution of $(x_1,x_2)$ given $X_3=x_3$

Question

Let $x=(x_1,x_2)$ be a trinomial distribution with parameters $(n;p_1,p_2)$. Deterine the conditional distribution of $(x_1,x_2)$ given $X_3=x_3$. Remember $X_3=n-x_1-x_2$. How do I write the distribution of $(x_1,x_2)$ and $x_3$.