I had a baby like that too. Very orally fixated!! One thing to do is to try to develop a routine, so you know if she is really hungry or not. Wake/feed, play, sleep. There are some that say 3 weeks is too early, but it really isn't. Teaching her to play after eating will help her stay awake during feeding, and that is really the key, to keep her awake. Whenever she seems to slow down, wake her up again. If you unlatch her, put your pinky or forefinger (nail side against the tongue) into her mouth. This is actually better than a paci, as she can take more in, similar to your nipple.
If she isn't hungry, she can be pacified with this and let your boobs rest. Then, let them get some air and some nice soothing nipple cream (Motherlove, or Earth Mama Angel Baby), will help. If she can be entertained and remain awake, for even 30 minutes, great, then put her down for a nap. When she is tired, she will most likely start rooting and show signs of what looks like hunger, but also signals tired. When she wakes up, she should be ready for more milk, and your boobs should have gotten a break.
During growth spurts, she may be hungry more often, but you still shouldn't have to feed for more than 30 minutes at a time. Also, as they get older, they get more efficient, and don't eat as long.