ArcGIS Server is not needed in order to share CityEngine Web Scenes. We recommend to share the Web Scene to ArcGIS Online in order to leverage the security and collaboration aspects of the arcgis.com site, as well as to easily reaching a global audience. However if those features are not required in your case the Web Scene can be shared using any web server by copying the Web Scene .3ws file into the web server public directory. Follow the instructions in the CityEngine help to learn how to do that.