Hi Iban,
I´m afraid you have wrong configuration on your bacula-dir.conf about storages...
I think you should have only one definition for storage, your autochanger named TSM3500. That way bacula will find correctly your storage device, the TSM3500, and the mtx-changer script and other configurations will help bacula to mount the volumes.
your "the storages contains" should be:
Storage {
Name = TSM3500
Address = 10.10.0.22
SDPort = 9103
Password = "xxxxxxxxxxxxxxxxxxx"
Device = ULT3580-TD3
Device = ULT3580-TD5
Media Type = LTO-3
Autochanger = yes
Maximum Concurrent Jobs = 1 # i
}