Unreal Engine Version 4
It is free. Tons of learning tutorials available. But, it does not import files in *.stl format.
To get stl files into Unreal Engine, you first need to download a free copy of Blender. Blender (a 3d model-building software) can import files in *.stl format.
Inside Blender, after importing an *.stl file, export it again into *.fbx file format. It is not difficult to learn a little bit of Blender - just enough to be able to import and export files.
Unreal Engine will read a file in *.fbx format.
Alternatively, locate a person who is willing to convert files from *.stl format into *.fbx format for you. I can do this, but do not have to time to make many file conversions. Could convert maybe five or ten for you.