Wow. You really need to read Healthy Sleep Habits Healthy Child. First of all, a three week old breastfed baby is only going to sleep a couple hours at a time anyway. From 3 weeks to 6 weeks you will notice a great period of alertness you havent' seen before. this is hormonal. By all means, LET your baby sleep when it needs to. That is one of the biggest misconceptions that the more you keep them up the longer they sleep. NOT AT ALL THE CASE. Sleep deprivation in a child of that age will raise cortisol levels (stress hormone) thus making her get LESS sleep. So, it is very natural for what your child is doing right now with being more alert. Let the baby sleep though during the day. The average child that age needs about 16 hours of sleep a day!!! In fact, she will only start to have scheduled naptimes at about 3 months old. The hardest time to get through is 3-12 weeks. The alertness will get worse and peak around 6 weeks, then it will gradually subside. Most babies will be sleeping much better at 3 months. At this stage, you should not be letting her cry. This is a really hard time for you and the baby at this stage, and this book really recommends doing whatever pleases your baby til you get to the 3 month mark. They are not "working" you at this stage, so letting her cry may just end up setting you up for failure. It is NOT a good idea til she has reached that 3 month time frame. It is very important for her to learn to trust you.
As far as breastfeeding, make sure you are keeping her on one side (right or left) as long as possible (at least 15 minutes) to make sure she is getting hindmilk. Usually, if they are getting only foremilk, they will be on you every 45 minutes or so, they will be very gassy, and their poop is a greenish cast. To make it simple, keep her on one side at least 15 minutes. Try to stimulate her if she is dozing by rubbing her back, toes, chin, or cheek. If she wants to sleep beyond this, just let her. Don't go to extremes to keep her up.
It is very normal at this stage for you to leak just before the feeding, particulary if she is starting to get fussy. The body will produce the milk even just with her crying. (It is a psychology thing called classical conditioning) Chances are you are producing enough milk. If she is going to the bathroom several times a day, you are fine. I think around this time my son was going number two almost every time he ate, and he always had a pee pee diaper. Not all kids are this way, and it changes the further along you get. A really good resource is the La Leche book, The Womanly Art of breastfeeding. I looked at that a lot til my son was about 6 months. You have to take some of it with a grain of salt, they are really preachy about breastfeeding and it turns some women off. Just use it as a guide about what you should expect at each stage.
In addition, you will notice as the baby gets older she will get far more efficient and empty the breast faster. So meal times will decrease. Do you have a lactation consultant at your pediatricians office? They are wonderful!