Uninstalling Lightroom 3 will remove all of the files necessary to run the program. But it will not remove the catalog or the images that have been imported into Lightroom. All of that information will still be available, and Lightroom CC will convert a catalog to the Lightroom 5 format by creating a new copy of the catalog.
Great, thanks. I just wanted to make sure. It would be very frustrating to lose all that information and organization!
What Jim says is correct, provided you are using the normal operating system process for uninstalling the application. Do not run about deleting files and folders.
With a Mac you simply drag the App file from the Application Folder into the trash.
On a PC go to the Control Panel and use the uninstall process.