I've just installed trial versions of ArcGIS Pro & CityEngine, in order to open a whole bunch of scene layer packages (.slpk) of city blocks from here:
What I want to do is to export the building models to a 3D format usable in apps like Sketchup, 3DS MAX or Blender, but ArcGIS Pro seems at best to have a very convoluted process. CityEngine has the export functions I'm after, but it can't directly import .slpk files. The city blocks aren't available in any other format except .slpk.
A scene layer package is a cached dataset so right now there is no way to convert the package to a different dataset. Scene layer packages can be viewed in Esri apps such as ArcGIS Pro, ArcGIS Earth, Scene Viewer and 3rd party apps such as ContextCapture, Vricon, Pix4D and Skyline Photomesh.