How can I configure RMAN to delete crontrol files automatically?
The RMAN Retention Policy for the control files and backup sets is defined as such:

(emphasis mine)

[CONFIGURE]..



  This is the command to start the configuration of the parameter


... RETENTION POLICY ...



  Specifies a persistent, ongoing policy for backup sets and copies that RMAN marks as obsolete, that is, not needed and eligible for deletion.
  As time passes, RMAN marks backup sets and copies as obsolete according to the criteria specified in the retention policy. RMAN automatically deletes obsolete backup sets and copies in the flash recovery area when space is needed. RMAN does not automatically delete obsolete files outside the flash recovery area: you must manually execute DELETE OBSOLETE to remove them.


... TO REDUNDANCY integer



  Specifies that RMAN should retain integer full or level 0 backups of each datafile and control file. The default retention policy setting is REDUNDANCY 1.
  If more than integer full or level 0 backups of a datafile or control file exist, then RMAN marks these extra files as obsolete. RMAN then determines the oldest of the retained backups and marks all archived redo logs and log backups older than this backup as obsolete. The DELETE OBSOLETE command removes obsolete datafile backups (full or incremental), control file backups, and archived log backups or image copies.


Applying the Above to Your Environment

In your case you have set the following:

CONFIGURE RETENTION POLICY TO REDUNDANCY 3;


Which means:


keep all full and level 0 backups for up to three versions [back]
keep all archive logs in-between these backups as well
keep all control file backups of each backup type as well


full and level 0 backup
archive log backup
when any autobackup of the control file is triggered



As you might notice, you could essentially have the same amount of backed up control files as you have the sum of: 


full backup files
level 0 backup files
archive log backup files


...or even more. (Read: Oracle 12c – Automatic Control File Backups for more details)

And if you are not backing up to the flash recovery area of your Oracle instance, then you would have to manually delete these files after backing up your database by issuing the following command:

DELETE OBSOLETE;


You could also test this by first running:

REPORT OBSOLETE; 


...to check that the control files in the directory are actually obsolete, and then manually delete them in RMAN with the command I mentioned before this one.

Summary

You could be seeing the following issues:


Everthing is OK, the instance has just created a lot of autobackup-ed control files.  
The Control Files are outside the FRE and have to deleted manually
You hit a bug (...read on below).


Because we don't know your exact backup scripts and other settings, it's hard to tell which point you are encountering, but you have a few good starting points.

Apparent Bug

There are cases (apparently) where the control file never gets marked as obsolete:


12.2 controlfile backup never gets marked as obsolete


A bug has been filed, and a fix supplied, but it doesn't seem to work in all cases. 

You might have to manually delete the control file backups in RMAN.

DELETE COPY OF CONTROLFILE LIKE '/mnt/backup/rman/xyz/&lt;part_name_of_control_file&gt;%';


Answering Your Question


  What can I do to delete these files also automatically?


Sorry, but that depends which situation you are hitting and can vary from: 


Will happen automatically
Can be deleted with REPORT OBSOLETE and DELETE OBSOLETE
Will only happen manually with DELETE COPY OF CONTROLFILE LIKE ...
Might work again after bug fix


References


Deleting RMAN Backups and Archived Redo Logs (Oracle Docs)
Configuring the Backup Retention Policy (Oracle Docs)
Listing Backups and Recovery-Related Objects (Oracle Docs)
Deleting Specified Backups and Copies (Oracle Docs)
12.2 controlfile backup never gets marked as obsolete (PATRICK'S ORACLE DBA BLOG)