If your dashboard is showing 3 users, then there are 3 users in the database, and that's probably the best place to delete the unneeded ones from.
Your hosting account may give you access to phpMYAdmin, an interface for editing your database. Before you do anything, be sure to backup your website including the database.
You may not know or be able to find your database username and password, but they will be in your wp-config.php file.
Very carefully, look for the users table. Each user will have a separate line in that table, and it should be pretty straightforward to delete the unneeded ones.