You can find the Mirror command button in the Modify panel on the Home tab in both the 3D Basics and the 3D Modeling workspaces, as shown in the following diagram:
The mirror command will work normally with 3D objects, but with limitations. You select the objects, and then you specify the mirror line as usual, regardless of the object position. The mirror line will be created in the UCS XY plane, and the mirror plane will be created by passing through that line and perpendicular to the XY plane.
It is impractical to use the mirror command ...