You can specify the mirroring plane by aligning the objects with a specified plane
or by specifying three points.

For example:

It is recommended that you use the gizmos available through the 3DMOVE, 3DROTATE,
and 3DSCALE commands to manipulate 3D objects.

The following prompts are displayed.

Select objects

Select the objects you want to mirror and press Enter.

Object

Uses the plane of a selected planar object as the mirroring plane.

Delete source objects

If you enter y, the reflected object is placed into the drawing and the original objects are deleted.
If you enter n or press Enter, the reflected object is placed into the drawing and the original objects
are retained.

Last

Mirrors the selected objects about the last defined mirroring plane.

Z axis

Defines the mirroring plane by a point on the plane and a point normal to the plane.

View

Aligns the mirroring plane with the viewing plane of the current viewport through
a point.

XY/YZ/ZX

Aligns the mirroring plane with one of the standard planes (XY, YZ, or ZX) through a specified point.

۳ points

Defines the mirroring plane by three points. If you select this option by specifying
a point, the First Point on Mirror Plane prompt is not displayed.