Thank you, dear Robert, for answering.
At last I solved it, by bypassing the problem -
indeed 3d scene does not have an option to display symbol by SVG path,
so, in order to display SimpleMarkerSymbol with SVG path (for its outline) within 3d scene,
I "converted" it to a polygon, dynamically computing the polygon ring points coordinates (using the SVG path, and scene scale), and it works for me.
I only have, as usual in 3d, to set the right "z" value, so it will be seen.
And polygon can be displayed in a scene.