Well there is a plugin available for using with Flex 3 (only for this version of flex).. to install follow:
Start by opening Help -> Software Updates -> Find and Install. This will open up a dialogue that will let you select to either update existing features or Search for new features to install.
Click the "New Remote Site" button and set Name to "Subclipse". Enter the URL http://subclipse.tigris.org/update_1.2.x and press ok.
If you just go with the default checked off, you'll also need to install some extra support libraries called Buckminister and Mylyn. but it is not req for normal use. so expand the subeclipse and uncheck integrational option.
Make sure that only "Subclipse Plugin" is checked and click "Next". Accept the terms, click "Next" and then "Finish" to install. This will start the download of the plugin. You'll need to verify the features installed and restart. That's it.
On the other side it requires a svn repository.. that can be created on a server on free webs like cvsdude, assembla etc...
in Flex3 go to windows --perspetive -- other and choose svn repository. it will open a new side bar. Right click in it and provide the path of your svn repository. now the project which is required to be added to svn just right click and choose team --share. (it will ask to choose svn or cvs in a new dialog, choose svn).