It is now more difficult to find Flex Builder 3 download. I just got a new laptop and wanted to install Flex Builder 3 because I am still working with projects that are based on it. Searched all over for the official download but had to hit a 3rd party site that referenced the link below.
Still works at the time of this post (6/14/10): http://download.macromedia.com/pub/flex/flex_builder/FB3_win.exe
Adobe is making it difficult to find past versions of Flex Builder as they want you to upgrade to Flash Builder 4.
Thank you Steven. A link to the Mac version follows: http://download.macromedia.com/pub/flex/flex_builder/FB3_mac.dmg