When you start feeing your baby cereal it's adding to the meal of milk, not replacing it. They still need their milk wether it's formula or breastmilk. One thing is when you start feeding them the cereal it should be feed to the child from a spoon and bowl. Don't mix or add cereal into their bottles. As for starting them at 3 months, I do feel that's a little too young as baby's tummies are not ready to digest solids yet. It can be hard on thier little tummies also could cause constipation or difficultity having a bowel movement. You do need to do your own research via doctors, pediatricians, books etc, as they may say differently. The guidelines change over time as well so need to look and go with the most recent one and as well as with your instincts and baby's. The Canadian Pediatric society say not to start solids until 6 months. I just started my baby at 5 1/2 months on cereal and they are loving it. I started him as he was nursing every 2 hours! So you do need to take in consideration of your baby. If your baby's eating fine and fairly spaced out and you don't really feel they need cereal yet, don't start. They don't need solids nutritionally until closer to 6 months. Hopefully that helped some more.