My command above is server-wide setting, which applies for the whole server will all existing mailboxes and for new created.
If you want to update a special score (3) only for all existing mailboxes, you can try to use the following command:
for i in `mysql -uadmin -p\`cat /etc/psa/.psa.shadow\` psa -Ns -e "select concat(mail.mail_name,\"@\",domains.name) as address from mail,domains,accounts where mail.dom_id=domains.id and mail.account_id=accounts.id order by address"`; do /usr/local/psa/bin/spamassassin -u $i -hits 3 -reject-spam true; done