That really depends on your setup. If you use the same database for three different galleries with different table prefixes, you shouldn't delete anything. From your question I assume that you've only one gallery. So I wonder why there are three different table prefixes. I assume you installed the gallery each time instead of upgrading it? Usually there's no need to delete old tables, as there aren't any. Have a look at include/config.inc.php to know which table prefix is used for your gallery. If you're certain that the other tables aren't used, you can safely delete them. Keep in mind that the table prefix is just a string, so a gallery running cpg1.5.x could also use the tables with the prefix "cpg132_".