Best practice integration of DSPAM into ma il system with separate Postfix and Dovecot VMs

Hello, please share an advice!
If I have separate Postfix and Dovecot VMs, should I:
1) install DSPAM as a completely separate 3rd VM (which I'd prefer),
2) or install it on Postfix VM,
3) or install it on Dovecot VM?
The confusing point for me is DSPAM retraining via Dovecot antispam and sieve. When everything is on a single VM, I can put in /usr/local/etc/dovecot/conf.d/90-plugin.conf:
antispam_backend = dspam
...
antispam_dspam_binary = /usr/local/bin/dspam
Here the last statement implies that DSPAM is on the same VM with Dovecot. But if I wanted to separate DSPAM completely, how should I accomplish DSPAM retraining? Or it is not a recommended setup - to have a dedicated DSPAM server?
P.S. For some reason, all HowTos in the Internet describe mail system setup with Postfix, Dovecot and DSPAM on the same VM. While it's useful as a working TEST setup, it gives no idea (and doesn't even come close) on how to implement a REAL setup with separate Postfix, Dovecot and DSPAM components.

Thanks in advance!


-- 
Орхан Касумов

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