Sounds just like my little guy - who will be 2 yrs old next month. Only my boy didn't sleep during the day either - he really didn't sleep much at all until 15 months.
But, there are definitely things you can do to "flip" his schedule around. Here are some mistakes I made in the evening hours & early morning hours: keeping him in bright or even dull light, talking to him, giving him eye contact, changing his diaper too much.
I highly recommend Healthy Sleep Habits, Happy Child by Weissbluth & Ferber's book Solve Your Child's Sleep Problems. Excellent resources for sleep because you will be dealing with various sleep issues for awhile. It took about 4 months before my son would fall asleep before MIDNIGHT...then at 6 months he would fall asleep by 10PM (but still waking up every 2-3 hours, sometimes every hour). At 7 months I finally did CIO using Ferber's method & within 3 nights he was falling asleep at 7PM - changed our lives dramatically!! Even though he was nowhere near sleeping "through the night" yet, it gave us time to ourselves, finally.
Every child is different, so yours may figure it out sooner than mine did. Currently my son is down to 1 nap a day, lasting approximately 90 minutes. Then he gets 10 hours of sleep a night - sometimes only 8 or 9 hours. And I don't anticipate that he'll be changing dramatically for a long while yet. He's up by 5:30, 6AM every single morning.
3 weeks is much too early to try & start "sleep training" - it becomes more effective once they hit 12 weeks, so he will probably change on his own in the next few weeks. Just try & hang in there, REST when he sleeps, keep the room dark when he's awake at night, etc. - another good resource is Happiest Baby on the Block - gives some good techniques for getting a newborn to sleep. Didn't work for me but I know other's have used it with success. :)
Good luck - I can really really empathize with you. Just take it one night at a time & stay consistent when you start implementing changes!