3. Given a geometric sequence $\left\{a_{n}\right\}: a_{1}=3$, and the geometric mean of the first to eighth terms is 9. Then the third term is ( ).
(A) $3 \sqrt[9]{81}$
(B) $3 \sqrt[7]{81}$
(C) $\sqrt[3]{9}$
(D) $3 \sqrt{3}$
3. B.

Calculating, we get $q=3^{\frac{2}{7}}$, thus, $a_{3}=3 \sqrt[7]{81}$.