After enabling the 3D extension on my wiki and uploading a 3D model, I found that there seems to be an issue with the extension when it attempts to load the thumbnail of that model. On the extension's MediaWiki page it lists 3d2png (a thumbnail renderer) as a dependency of the extension, so perhaps this is what is causing the issue?
If it helps, the error message I get when I try to load it is: "Sorry, the file cannot be displayed[.] There seems to be a technical issue. You can retry if it persists. Error: could not load image from https://static.miraheze.org/funkeywiki/thumb/8/85/FunKey_Button_Cover_3D_Model.STL/800px-FunKey_Button_Cover_3D_Model.STL.png "
I could try testing it with other models if that might help?