What is the simplest way to get a Rails 3 edge app started?

(optional) Install and configure RVM (Ruby Version Manager) this will allow you to isolate a Ruby installation from the rest. Rails 3 is optimized for Ruby 1.9, so this will allow you to try it out in the preferred VM.

• Install the gem bundler: gem install bundler
• Create a new dir for your sample app: mkdir testapp && cd testapp
• Create a file called Gemfile in your the directory with the following contents:

testapp/Gemfile

gem "rails", :git => "git://github.com/rails/rails.git"
git "git://github.com/rails/arel.git"
git "git://github.com/rails/rack.git"

source :gemcutter

#any other gems you need
gem 'sqlite3-ruby'

Run the following commands:

$ bundle install
$ bundle exec rails .
$ bundle exec rails server 

(note, rails will try to rewrite your gemfile, tell it to ignore the change)