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