If you want to create a quarterly recurring appointment in Outlook calendar, please do as follows:

1. In the Calendar view, please click Home > New appointment to create a new appointment.

2. In the opening new Appointment window, please click Appointment > Recurrence. See screenshot:

3. In the Appointment Recurrence dialog box, please do as follows:
(1) In the Appointment Time section, please add start time, end time, and duration for each occurrence of the entire appointment series;
(2) In the Recurrence pattern section, please check the Monthly option, and then specify the frequency of every 3 months, such as The last weekday of every 3 month(s) or Day 29 of every 3 month(s);
(3) In the Range of recurrence section, please specify the recurrence range as you need;
(4) Click the OK button.

4. Now you return to the Appointment Series window, please add subject, location, compose appointment note, and click Appointment Series > Save & Close button.

So far you have created a quarterly recurring appointments which occurs every 3 months in Outlook.

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