I have been trying to do the same thing, but had no luck. I followed this link arcgis-pro-sdk-community-samples/Map-Authoring/ExportWeb3DObjectResource/ExportWeb3DObjectResource a... but it didn't worked for me. I have building footprints layer with height info, I was able to extruce and symbolize in ArcGIS Pro, but export tool doesn't work for me.
Do you know what rule package you used to symbolize in ArcGIS Pro?
Any help appreciated!!