Chọn phương án C.

\(\begin{aligned}
3^{2x}-2\cdot3^{x+2}+27=0\Leftrightarrow&\,\left(3^x\right)^2-2\cdot3^2\cdot3^x+27=0\\
\Leftrightarrow&\,\left(3^x\right)^2-18\cdot3^x+27=0\\
\Leftrightarrow&\left[\begin{array}{l}3^x=t_1\\ 3^x=t_2\end{array}\right.\\
\Leftrightarrow&\left[\begin{array}{l}x=\log_3t_1\\ x=\log_3t_2\end{array}\right.
\end{aligned}\)

Theo định lý Vi-ét, ta có \(t_1\cdot t_2=\dfrac{c}{a}=\dfrac{27}{1}=27\).

Khi đó, $$\begin{aligned}
x_1+x_2=&=\log_3t_1+\log_3t_2\\
&=\log_3\left(t_1\cdot t_2\right)\\
&=\log_327=3.
\end{aligned}$$