your max HP is determined by by whatever 3 foods you have eaten, and this amount is temporary, depending on the timer each individual food has. Once you have eaten 3 foods and your max HP is established, that's what you have to work with until the timer(s) run out.

IF you take damage, you can use a potion to restore a set amount of damage (whatever amount the potion states), gradually, until your hp is maxed again or the potion amount is reached.