Try Microsoft 3D builder, it comes preinstalled on Windows 10. Microsoft is going hard on 3D. It is pretty easy, just import your STL files to prepare your buildplate where you can rotate, scale, copypaste STLs and export it as a single STL. Just remember about your buildplate limits. Then "save as" and don't forget to select *.STL as .3mf is preselected by default and not compatible with astroprint jet. Last step is upload to astroprint and thats it. I bet this feature should come soon as they built a 3D viewer few days ago. They are doing it awesome!