Hi,
You are quite correct. Backup Exec ships with 1 license for a library. It's per drive. If you have an autoloader, you don't need a license. If it's something like an MSL/ESL/EML-e, then the LEO gets installed on the server with those drives. If it has 2 drives, you install a license on that server.
Those licenses get put on your server that connects to that library, and you add the number of licenses for the number of drives you have...in this case those additional 3.