Here's some links for you:
http://www.drgreene.com/21_118.html
http://kidshealth.org/parent/growth/feeding/breastfeed_of...
http://family.go.com/parentpedia/baby/care-basics/baby-bo...
The frequency at which a newborn poops, varies.
Keep in mind, that a baby's internal organs are still developing as well...it takes much of their first year and then some, to fully develop. So, they are adjusting to a lot of things.
Sometimes, something the Mom is eating can also affect the breastmilk, and cause upsets in the baby's system.
Is he having gas problems? This can cause crying too. Or is it constipation? Or perhaps he will poop, but not yet.
Do you know FOR SURE that he is crying because he has not pooped? A newborn cries... sometimes for no reason, as they have a lot to adjust to in this world. Sometimes they cry because they are hungry. A newborn and baby should be fed/nursed on demand. This can range from every hour, to 2 or 3 hours. But being that he is only 3 weeks old, I would not go that long without a feeding, day or night.
Also, you may want to make sure that your milk flow is producing enough milk? Sometimes this can cause them to cry, because they may not be getting enough intake.
If you are using formula in conjunction with breastmilk... make sure this is okay with your Pediatrician. Also, if you are "replacing" a breastfeeding with formula...this CAN affect your milk production and lessen it possibly.
Instead, if using formula...many times, it is given after a breastfeeding...as a supplement. That way, it won't interfere as much with your breastmilk production. Is it possible for you to just do exclusive breastfeeding?
Maybe your little guy wants to nurse and breastfeed, versus a bottle. Although little, they can have preferences already. It's their survival instinct.
Well sorry for rambling... just some thoughts. A newborn crying can be due to many reasons. See what your Pediatrician recommends.
Is he having enough wet diapers as well?Sometimes, if they are not intaking enough, they will not have as much wet diapers.
I'm sure he will poop... and it might be a huge one! But check with your pediatrician. If he is growing and gaining weight appropriately... then he's probably getting enough intake and nutrients.
Sometimes, bicycling their legs, and gently massaging their tummy in a circular manner can help.
I wish you the best, sorry I rambled, just some spur of the moment ideas.
Take care,
~Susan