Do i need to wake up my 15 days old girl for her feedings? she started sleeping more than 3 hours.

Sleepy baby: As long as she is a full term baby, with no medical problems, better to let her sleep. Wake max 4 hours in daylight hours, never wake her at night. :)

Not necessarily.: If she's gaining weight well, you do not need to wake her up every 3 hours. Most babies her age do wake up during the night after 4 hours or so. If she's sleeping longer than 4 hours during the day, or 5-6 hours at night, I would recommend waking her up. You should discuss this with her doctor also.