What are you trying to do with the 3d sketch?
You cannot mirror or pattern 3D sketches. If the sketch is simple enough then a workaround is to fill or extrude the sketch a short distance to form a surface body that has edges along the sketch entities. Then you can mirror and pattern the surface body. Finally you can start a new 3D sketch and convert edges from the mirrored/patterned surfaces.
And if the sketch isn't simple enough ?
I will have to construct simple enough surfaces, to intersect them, and then to mirror and to pattern with the curves, i suppose ...
Thanks for your help.