Hello Nikolaus,
For the time being , you do not have the option to disable 3DES yourself. However, I created a private issue tracker (sent privately) to investigate the possibility if the backline team can disable it for your custom domain.
Meanwhile, if your goal is to create a Payment Card Industry Data Security Standard (PCI DSS) compliant environment, you may check this document for best practices.
As to update this community thread, working with Nikolaus, and the Engineering team we were able to disable the cipher (3DES) for his custom domain.