Azure pipelines cron every three months

I want to run my azure pipeline every three months, but for some reason my schedule is not set. If i specify a shorter cron schedule, it does appear.

When i set it every three days a schedule does appear

  - cron: "0 0 */3 * *"
    branches:
      include: 
        - master
        - azure-pipelines
    always: true

But when i try to cron every three months no schedule is set. I have tried the following cron settings:

- cron: "0 0 * */3 *"
    branches:
      include: 
        - master
        - azure-pipelines
    always: true

&

  - cron: "0 0 1-7 */3 SUN"
    branches:
      include: 
        - master
        - azure-pipelines
    always: true

Am I doing something wrong? or is it not possible to schedule over longer periods? Thx!