You must remove the QuickTime Plug-In v1.1 installed in your internet browser plug-in folder by QuickTime 2.1.2 prior to installing QuickTime 3.0. If you do not remove the older plug-in, your browser may use the old version instead of the newer version.
You do not have to uninstall the older version of QuickTime prior to installing QuickTime 3.0, just the QuickTime Plug-in.