Let him sleep. He'll eventually adjust to a normal schedule on his own. Right now he's just sleeping a lot but within a few weeks or months he'll slowly start being awake more and will learn to adjust to your schedule.
At only three weeks you can't do anything really but Let them sleep. There's a reason newborns sleep so much, as they get older they will start to eat more at a time but less often and gradually they'll learn to sleep for longer periods of time. Eventually they'll sleep through the night and nap on and off during the day. It's hard but you learn to live with it, sleep when they sleep kwim? You can't make the baby stay up all day and you shouldn't try to do so either. Let the baby sleep when he needs to.