If you are using ruby 1.8.x all you need to do to install rails_code_qa in your rails 3.x app is add the following line to your Gemfile.
If you are using ruby 1.9.x you will also need to add the following to your test/test_helper.rb file
require 'simplecov' SimpleCov.start 'rails'
Rails Code QA takes care of picking which code coverage tool to use based on the version of ruby you are using. If you are using ruby 1.8.x code coverage is calculated using rcov and if you are using ruby 1.9.x coverage is calculated using simplecov.
Static Code Analysis
Rails Code QA also runs flog and flay to help you find other issues in your apps code.