Opening a .3mf without a slicer

Rename it to .zip and any unzip tool opens it, because that is what it is. Or drop it on preslice and read the print settings, filaments, plates and mesh in a browser tab, with nothing to install.

Why does renaming it to .zip work?

Because a .3mf is a ZIP archive with a different extension. The 3MF Core Specification defines the format as an OPC package, and an OPC package is a ZIP file with rules about what is inside it. Nothing is encrypted and nothing is proprietary.

Unzip one and you get XML: 3D/3dmodel.model holding the objects and where they sit, _rels/.rels pointing at it, and whatever the slicer wrote beside it. What a .3mf contains walks the package part by part.

What can I read once it is open?

The geometry and the placement, from the model part. Every object, its triangles, and the transform putting it on the plate.

The print settings, if a slicer wrote them, from a config part beside the model. Those are not in the specification: each slicer invented its own. PrusaSlicer writes INI to Metadata/Slic3r_PE.config. Bambu Studio, OrcaSlicer and Creality Print write JSON to Metadata/project_settings.config. Cura writes its settings into the model XML itself. Print settings in a .3mf has the layout for each.

What you will not get from an unzip tool is a picture. The model part is a list of vertices and triangles, and reading it as text tells you very little about the shape.

What does opening it in preslice add?

The parts an unzip tool leaves you to read by hand. It draws the mesh with every object placed where the file says it sits. It names the slicer and version that wrote the file, and the filament in each slot with its colour. And it puts the print settings under their real key names instead of raw XML.

It also runs six security checks first, which matters if the file came from a stranger. And the Raw tab still gives you every member as text, with search across all of them, so nothing is hidden behind the friendlier view.

Parsing runs in the browser tab. The file never uploads.

Which programs open a .3mf directly?

Every slicer that writes one: PrusaSlicer, Bambu Studio, OrcaSlicer, Cura, and Creality Print all open a .3mf for its geometry. What they do not reliably do is read each other's settings, because those live outside the specification. The slicers covers what each one writes.

Windows ships a 3D Viewer that opens one for the shape alone. Most CAD tools import 3MF now, and lib3mf is the consortium's reference reader if you are opening one from code.

What if it will not open at all?

When a file will not open lists the usual causes: a truncated download, a file that is a .gcode.3mf rather than a project, or a package whose geometry lives in parts the reader never fetched.