STL and 3D CAD viewer options?

Can anyone recommend a fast 3D model viewer (mainly .STL files) for Leap 15.3? I have thousands of STLs that I often need to look through and I don’t want to have to fire up FreeCAD or similar, just to view them. The 3D file viewer that comes with Win10 is the sort of thing I’m looking for.

I could almost live with a thumbnail previewer in Dolphin but can’t find one that is easy to install.

I have installed Cura but not having any luck in running it (gets to ‘Loading GUI’, then quits).