For big projects that involves more people a version control software is a must. There are many but from the two most used, which are CVS (Concurrent Versions System) and SVN (Subversion), I’ll explain how to install Subclipse on Flex Builder 3 - Subclipse is a plug-in providing support for Subversion within the Eclipse IDE (Flex Builder is based on Eclipse IDE).

Starting with the Flex Builder opened do the following actions:

1. Help menu → Sofware Updates → Find and Install → Search for New Features to Install → Check Europa Discovery Site (do not check automatically select mirrors) → Finish
2. Select a mirror from the List that appeared → Ok
3. In the new list expand Europa Discovery Site → expand Java Development → select Plug-in Development Enviroment and Java Development Tools → Next → Accept the terms and conditions → Next → Finish
4. Restart Flex Builder
5. Help menu → Sofware Updates → Find and Install → Search for New Features to Install → New Remote Site → Enter the following: name - Subclipse, URL - http://subclipse.tigris.org/update_1.0.x/
6. Then a new entry should appear in the list with the list with sites for search → Expand Subclipse → Expand Subclipse Plug-in → Check Subclipse 1.2.x (available version might be different when you read this) → Next → Accept the terms and conditions → Next → Finish
7. Restart Flex Builder

These should be the necessary steps to install Subclipse on Flex Builder 3 but there is a small possibility that I overpassed minor steps.

Other resources can be found here: onflexwithcf, nodans