"Studio 3 simplified greatly the concept of adding JavaScript content assist."
Typical ESRI -- by 'simplify' they mean making things 5 times more difficult, and basically unusable!
Unfortunately we do not have a Code Assist plug-in for any version of the API that works with Aptana Studio 3. At version 3, Aptana changed the way Code Assist works and there have been some technical challenges with building Code Assist for version 3. We are working to resolve these issues and hope to have an update soon.
3 years later and we're still waiting for the new code assist for Aptana 3?