Custom set reminders
Assuming you've already set it to repeat every 3 days, you can do any of the following:


within the Reminders app, tap the radio button next to the reminder to mark it as completed
if you have an alert that appears on the Lock screen, swipe left and select Mark as Completed
if you have a device that supports 3D Touch (e.g. an iPhone 6s and above, but not presently any iPad models), you can also hard press and select Mark as Completed


Regardless of the option above, the reminder will repeat as per the schedule you've set.