The correct answer is $\boxed{-2}$.
The sum of the roots of a polynomial equation is equal to the negative of the coefficient of the $x^2$ term divided by the coefficient of the $x^3$ term. In this case, the coefficient of the $x^2$ term is $-6$ and the coefficient of the $x^3$ term is $1$, so the sum of the roots is $-\frac{-6}{1} = 6$. We know that two of the roots are $1$ and $3$, so the third root must be $6-1-3 = \boxed{-2}$.
The other options are incorrect because they are not roots of the given equation.