If they are internal and permanent (read: unlikely to be removed on a daily basis), I’d just mount them based on their purpose and not them being separate HDDs physically. If they are meant for logs, mount them at /var/log. If they are meant for your movies, /home/user/data is more than fine. In general FHS describes the directory hierarchy, not which parts of it are mountpoints and which are physically on the same media. Technically you’re fine having each and every directory on a separate HDD.