I'm assuming this is more of a daycare center than a school, even so, they have to let you know what you are eating due to the potential for allergies.
If the center makes their own food, ask the cooks for a menu. If they don't have a nutritional breakdown, you'll have to input the info into a website such as calorieking.com. If the lunches are brought in or catered, then they should have a menu with a breakdown of nutritional info.
If nothing else, eat what the three year olds eat in the same or smaller portions.
Good luck!