You mean your baby poops every 3 days? I think that could be normal, but if she's not eating any solid food you should try and give her some. At 8 months just small amount like a few tablespoons are good enough. If she doesn't like something maybe try something else? Fruit or veggie puree, porridge, cereal etc. You can mix some milk in baby cereal so it's more runny.
Ansu Dubey