3. For sets $A$ and $B$, the union $A \cup B=\left\{a_{1}, a_{2}, a_{3}\right\}$. When $A \neq B$, $(A, B)$ and $(B, A)$ are considered different pairs. The number of such $(A, B)$ pairs is
A. 8
B. 9
C. 26
D. 27
Answer
As shown in the figure, the set $A \bigcup B$ is divided into three parts: I, II, III, where $a_{1}, a_{2}, a_{3}$ can be independently placed into the above three parts. Therefore, the placement methods for $a_{1}, a_{2}, a_{3}$ are 27, meaning there are 27 pairs $(A, B)$ that satisfy the condition. Therefore, the answer is $D$.