Just to verify, you have uninstalled version 2 before installing version 3? Are you installing on the Administrator account or just an account with administrative privileges?
Do you have multiple versions of Dreamweaver installed? Make sure you are renaming the configuration folder/deleting the winfilecache for the correct version of Dreamweaver. If you renamed the configuration folder for the version of Dreamweaver you're installing the extension on, the previous version should not load up.