3 2 Hint: $E F=B C=3, \triangle F A^{\prime} B^{\prime}$ divides the tetrahedron $A^{\prime} B^{\prime} C D$ into two tetrahedrons $C F A^{\prime} B^{\prime}$ and $D F A^{\prime} B^{\prime}$ of equal volume, with heights $C F=D F=$ 1, and base area $S_{\triangle F A^{\prime} B^{\prime}}=3$, then the volume
$$
V_{A^{\prime} B^{\prime} C D}=2 V_{C F A^{\prime} B^{\prime}}=2 \cdot \frac{1}{3} \cdot 1 \cdot 3=2 .
$$
(3) In rectangle $A B C D$, $A B=2, B C=3, E$ and $F$ are the midpoints of $A B$ and $C D$ respectively. With $E F$ as the axis of rotation, rotate $\triangle F A B$ in space by $90^{\circ}$ to $\triangle F A^{\prime} B^{\prime}$. Then the volume of the tetrahedron $A^{\prime} B^{\prime} C D$ is $\qquad$.