Open Lightroom using the catalog that you want to be your master catalog. Then go to the File menu and choose import from another catalog and choose the other ones. Those images and all the work that has been done will be imported into your master catalog. I strongly recommend that you create a backup of your master catalog before you start into this process. After you have completed this important process for all the other catalogs, you will be able to delete the other catalogs and their associated previews folders.