How to set cron job which runs every three days?

After logging in, click on “Add new cron job”, enter a cron job name (optional), choose following configurations in time setting dropdown boxes:
Minute: any number from 0 to 59
Hour: any number from 0 to 23
Day: */3
Month: *
Weekday: *

Enter the URL you want to call and choose the rest settings if necessary, click on “Create” button, wait until the notice popups, you're done.
The final time pattern will be "(the number you set for minute field) (the number you set for hour field) */3 * *" after the cron job is added.