Did you try to do the mirror for 3D Sketch before?
In SolidWorks 2018 onwards, users can able to mirror 3D Sketch Entities. Below article we have attached a steps to do a mirror for 3D Sketch Entities. Kindly look at it.
2. Click Mirror Entities > Select 3D Sketch Entities to Mirror and select a Plane or Planar surface in Mirror About as like below.
3. After mirroring 3D Sketch Entities, we able to do a feature like below.
By using these technique (3D Sketch Mirroring) we can able to modelling faster and more efficiently.