I have a model that consists of 4 parts. There are also 2 .stl files that do not have supports. For conversations sake, here is my list of files.
Front with supports.3mf
Right with supports.3mf
Left with supports.3mf
Rear with supports.3mf
Right.stl
Left.stl
What happens if you enable supports in a .3mf file that already has supports built into the model?
Conversely, can those supports be easily removed from the .3mf file.
The removal of the supports in the .3mf file took substantially more muscle to break loose than in the .stl files where I let Bambu Studio create the supports by enabling supports in Bambu Studio.
Not sure if you do this, but look at Specification - 3MF Consortium
The 3mf file is a zipped file. You can change the extension to .zip, and unzip it into its component parts. Manufacturers, such as bbl, do not always adhere to the ‘official’ structure of the file format, but you may find that the supports are in a separate file to the main stl file for your two missing stl files. If not separated, then you will need to remove the supports in cad, most likely
I think you will find that Studio will only add support where necessary, so it should not add supports where they are already designed into the model.
With a good deal of effort you can probably design supports in the model that are easily removed, but it is much simpler to use the support-specific settings in Studio, which has controls for support wall thickness, density, distance, interface spacing, etc. Those settings can greatly influence ease of removal, and it is difficult to replicate their effect separately from the model settings.
I would only create supports in the model after Studio proved it could not find an acceptable solution.
EDIT - I may have misinterpreted the question. If you downloaded a 3MF with “built-in” supports, it may be a model that had supports as part of the original CAD design. The supports will be visible in the Prepare tab of Studio before slicing. Removing these can be difficult if you do not have access to the CAD files and program. You might be able to get rid of them with creative use of the cut tool and negative primitives, and then let Studio add more easily removed supports.
It may also be a model that originally had NO support, and then supports were enabled as part of the project settings. These are not seen until after slicing and can be turned off or adjusted as you wish.