3MF to STL

preslice writes STL from the meshes it has already read, so the export happens in the tab and your .3mf is left alone.

How do I save one object as STL?

Open the Content tab and find the Mesh objects section. Each object has an Export STL button, and it saves that object's triangles under the object's own name.

How do I save every object at once?

Export all sits in the Mesh objects header. It writes one ZIP named after your file, so benchy-plate.3mf gives you benchy-plate-stl.zip with one STL per object inside. Two objects called Body become Body.stl and Body-2.stl.

What does STL throw away?

Everything except the triangles. An object's name survives as a file name; its colour, its place on the plate and every print setting around it do not. See 3MF against STL.

Is it ASCII or binary STL?

Binary. It is the smaller of the two forms and every slicer reads it. An STL keeps no units, so the millimetres the 3MF Core Specification records on the model are gone from the copy.

Can preslice write STEP, OBJ or SVG?

No, and the reason differs for each. STEP holds exact surfaces while a .3mf holds triangles, so something has to rebuild the solid: Autodesk Fusion and FreeCAD both do that from an imported mesh. For OBJ, Blender reads a .3mf with the 3MF Import/Export add-on and writes one. SVG is 2D artwork, so there is no version of your model to write into it.

Why keep the .3mf then?

Because the STL is a copy with the thinking removed. The .3mf still holds the plate, the filament choice and the settings, and you can send it to another slicer later.