Having a bowel movement only every 3 days is not uncommon for a breastfed baby, but I would be more concerned about the gas. Although she is enjoying it, it may be that she is a little young to be able to cope with digesting the solids. I would suggest stopping the rice cereal and going back to just breastmilk for a few more weeks, then trying again and see if she has less gas and more frequent poos.