3. As shown in Figure $1, \Omega$ is the geometric body obtained after the rectangular prism $A B C D-A_{1} B_{1} C_{1} D_{1}$ is cut by the plane $E F G H$, removing the geometric body $E F G H B_{1} C_{1}$. Here, $E$ and $F$ are points on the line segments $A_{1} B_{1}$ and $B B_{1}$, respectively, different from $B_{1}$, and $E H / / A_{1} D_{1}$. Which of the following conclusions is incorrect? ( ).
(A) $E H / / F G$
(B) Quadrilateral $E F G H$ is a rectangle
(C) $\Omega$ is a prism
(D) $\Omega$ is a frustum
3. D.

Since $E H / / A_{1} D_{1}$, therefore,
$E H / / B C, E H / /$ plane $B C C_{1} B_{1}$,
$F G=$ plane $B C C_{1} B_{1} \cap$ plane $E F G H$.
Thus, $E H / / F G$.
It is also easy to see that quadrilateral $E F G H$ is a parallelogram, and $A_{1} D_{1} \perp E F$, so $E H \perp E F$.
Clearly, $\Omega$ is a prism. Therefore, $\Omega$ is not a frustum.