3 weeks is a growth spurt and odds are that things will change either now or shortly...and then just as you get used to a new schedule they will change again at 6 weeks, a little maybe at 90 weeks, and often a HUGE change around 12 weeks. Odds are pretty good that he'll be letting you know he's hungry during daytime hours as well very soon now.
I wouldn't let him sleep *too* long in the daytime, but I also wouldn't stick to a 3-hour schedule like clockwork either. I know this may sound like the last thing you want to hear, but "fake it till you make it" ended up being my mantra, especially with my second, whose entire personality and schedule and sleep needs were so very different from those of my first.