You may want to see how TSA is handling it these days b/c baby food is exempt from the other rules around liquids/gels, but when I flew with my daughter as an infant, you could bring formula and baby food in a carry on, but only a day's worth; and if you don't have an infant with you, they might question it because a day's worth for a 3 year old is probably going to be different. I brought a couple of the square plastic containers and packed the rest. Check their website or maybe even ask the pediatrician as they may have dealt with it with other parents. It sucks, but there are a lot of discretionary calls made at the gates.