In Studio 3 much of this sort of functionality is still coming as Aptana is going more of a TextMate bundle route for a great deal of their features, which I think is really cool.

That said, there is already a jQuery bundle. The jQuery bundle, if you don't have it installed yet, can be installed from Aptana via the Commands menu (Commands->Bundle Development->Install Bundle and then select jQuery from the list).

This support discussion has some details about jQuery code assist in Aptana: https://aptanastudio.tenderapp.com/discussions/problems/2006-studio-3-jquery-support-is-lacking

In particular, it mentions adding a jquery vs doc file (http://code.jquery.com/jquery-1.4.1-vsdoc.js) to your project. This seems to be the key.

If you install the jQuery bundle and add the aforementioned JS file, you can get jQuery code hints in your Aptana editor(s). I just ran through this in one of my Rails projects and it does work (I'm on the latest beta of Studio 3 for Mac).

This is incorrect. While this will give you jQuery snippets and some commands, to actually add jQuery content assist, all you have to add is a single file. Please follow the instructions here instead:

http://wiki.appcelerator.org/display/tis/Using+JavaScript+Libraries

to have code assist with aptana 3 follow this instructions https://wiki.appcelerator.org/display/tis/JavaScript+Library+Support (i.e. Commands-> Bundle dev->Install-> select jquery) and put your jquery.js library in your project as usual (drop it in a folder of your choice)