You have installed RoR3 and now you want to install a rails3 webshop.
I think the best solution is Spree. This is the only rails3 based e-commerce solution at this time.
How do you install this shop?
rails new rails3-shop
edit Gemfile and add new line
Install new spree site.
rails g spree:site
The rails server command starts the webrick server. You should go to http://localhost:3000/ to check spree.