This is not exactly an apples to apples comparison. HOOPS Web Platform (and HOOPS Communicator) consists of three primary components. HOOPS Converter which converts CAD formats into streamable 3D content, HOOPS Server which then streams that 3D content to clients and then the HOOPS Web Viewer which is a engineering-focused 2D/3D viewer. Three.js is most comparable to the rendering engine within three.js. The difference between these two rendering engines is that the HOOPS Communicator engine is an engine which was built from the ground up to deal with engineering data. This means that it can handle 2D and 3D, has fast selection, supports text and has fast rendering speeds with the high triangle count models you typically find in engineering datasets. Three.js is a great technology and can be used to build engineering technology but to provide good performance and engineering functionality (e.g. cap plane generation, measurement) requires additional development.