The 319 has a built in cam for the 3 step zig though #2 is available for the 306's and it's possible the 401(500 series too) and that series has it. I believe they do. I know my vintage Elnas have the cam for it as well. I've used it to do bindings as it's a bit more decorative and a regular zig zag can create a bit of a tunneling effect without proper stabilizing or if the zig zag is too close.
I would bet there's a 3 step or multi-step zig zag cam available for the Necchi since so many other machines have it.