How to run systemd timer every three hours

The syntax to run a systemd timer every three hours is:

systemd_oncalendar_every_3_hours.conf

OnCalendar=*-*-* 00,03,06,09,12,15,18,21:00:00

OnCalendar=*-*-* 00,03,06,09,12,15,18,21:00:00

i.e. run it on the first minute and first second (00:00) of every second hour (00,03,06,09,12,15,18,21).

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