If it is for a one-off wake-up at 3 am, you can try going to bed early, around 6 - 8 pm, to ensure you get 7 - 9 hours of sleep. If you plan on getting up at 3 am frequently, you should develop a routine that prepares your body clock for sleeping and waking up on a schedule. Tablets like B・SYNC ON can help you wake up naturally without an alarm.