RubyGem version error: activesupport(1.4.4 not = 3.0.0.beta) (Gem::LoadError)

I want to use the last version of rails, in my Gemfile: gem 'rails', '3.0.0.beta3'

When trying to start my server I get this error, do you know how I can fix this issue?

$rails s
/Library/Ruby/Site/1.8/rubygems.rb:777:in `report_activate_error': RubyGem version error: activesupport(1.4.4 not = 3.0.0.beta) (Gem::LoadError)
    from /Library/Ruby/Site/1.8/rubygems.rb:211:in `activate'
    from /Library/Ruby/Site/1.8/rubygems.rb:246:in `activate'
    from /Library/Ruby/Site/1.8/rubygems.rb:245:in `each'
    from /Library/Ruby/Site/1.8/rubygems.rb:245:in `activate'
    from /Library/Ruby/Site/1.8/rubygems.rb:1056:in `gem'
    from /usr/bin/rails:18
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bundle install return Your bundle is complete!

gem list give me

activesupport (3.0.0.beta3, 2.3.5, 2.2.2, 1.4.4)

---

ok so you have activesupport 3.0.0.beta3 while it requires activesupport 3.0.0.beta. Try this:

gem install activesupport -v 3.0.0.beta --pre