its common frnd every smartphone will lose plenty of battery percentages during even if u kept phone under safe mode, because android systems idle process needs the battery...
Device have to stay awake for your Alarms, Call and other activities. You lost only 3% of battery, it doesn't mean it is damaged. Instability in network causes battery drain which is common. So don't worry.
If the issue still persist, kindly register your concern in Samsung Members app (Samsung Members app > Get help > Feedback > Error report/ask question). Kindly send us the screenshot (which apps are consuming most of the battery). Note: Please send device log files within 15 minutes of issue occurrence along with the exact issue details.
