Actually, you can have all three. The way they work is based on the icons at the top of the screen. Au Lait items make the little "HP" symbol, juices give the "MP" symbol, and food items show up as a hamburger. So, you can have any three items at the same time, as long as none of them use the same symbol. Unfortunately, spells like Refresh and Regen use the same symbols as Au Laits and juices, so they DO NOT stack with their respective food items. By the way, combining all three is a huge help when leveling weak jobs : )