I want to use Aptana Studio 3 Beta for all my Ruby developments. I am planning to write a Windows appl. using FxRuby framework, but how to integrate this framework with Aptana Studio 3?
I'm not 100% sure I understand what exactly you're looking for here. Studio 3 will index your ruby core, std lib and gems installed on whatever version fo Ruby is on your PATH. It uses that index to generate code assist. As long as you have the fxRuby libraries built and installed into your system, we should pick up the ruby API and offer code assist for you.
Here's the relevants ection of fxRuby's wiki on building their library on windows: https://github.com/lylejohnson/fxruby/wiki/Setting-Up-a-Windows-Build-Environment
I will say a word of caution: installing, building and generally dealing with Ruby binaries on Windows is a difficult task unless you're comfortable with MinGW.