Just use the CS3 Extension manager (not the CS4 ext. manager) to install the "old" extension. You will be given the option to apply the extensions to Dreamweaver CS3. I have used this method to run Dreamveraver 8, CS3, and CS4 on the same machine. The key is ...use the appropriate Extension Manager for the corresponding Dreamweaver version. You should also check the CS4 extension manager after the installation is complete to verify that the old extension is not active for Dreamweaver CS4 as well.
