Reminders get sent out automatically based on your Reminder Schedule. You do not need to trigger a reminder to send out.
You can change when reminders get sent in Settings. You can set them to go out any number of days or hours ahead of time. You can set a reminder to be sent from 30 days to 1 hour before the appointment. Reminders will always be sent from your local time zone.
If you want to send out your reminders exactly 72, 48, or 24 hours ahead of the appointment, then you can switch your Reminder Schedule to send out reminders 3, 2, or 1 day(s) before the appointment.
See this article for info on sending out more than 1 reminder.