K.T.
I read the other responses and it is normal for a breastfed baby to go up to a week without going, but 3 does seem like too long to me. I would really pay attention to the consistency of the poop. If it is hard then she is defintely constipated. If it is soft, yellow, and seedy then maybe she just needs to eat from you more often. The more she takes in the more likely something will come out. Maybe she's using all of the food she gets for the nutrients. If I were you these are the things I would consider...1.Daily infant massage to stimulate her system, 2.Try some Chiropractic, I've seen some amazing changes take place with infants and children. 3.Consider starting her on veggies rather than rice cereal once you go to solid foods. Avacados are a good one to start with and sweet potatos also. I just know that rice cereal has very little nutritional value and it may add to her problem. Veggies have more fiber and be a better choice. Hope this helps.
Good luck!