In Outlook, click File>New>Appointment. Enter whatever data about that
appointment you wish. Check the "Reminder" box and choose a reminder time.
Then click Recurrence. Select "Daily" and "Every X days". Specify 84 for
X. Or select Weekly and specify 12 for "Recur every X week(s) on" and
choose a day. Or select Monthly and "Day X of every Y month", choosing a
likely day for X and "3" for Y.