Powdered Formula and Flying?

How should a package my daughters fomula when traveling on a plane? I figure I will need about 3 bottles worth(one before flight,one during, and one after). Its only a 3 hour flight but I want to have more then not enough. I usually use the containers that you can premix the formula for three feedings. I don't want TSA to think its not formula though so I figured I should take the container. Should I have the container already opened or keep it shut(seal still intact)? If you say keep it open should I only have a little in the container or have a full container?